The below are the simplest possible descriptions of various symbols,
just to help you keep reading if you do not remember/know what they
stand for.
Watch it. There are so many ad hoc usages of symbols, some will have
been overlooked here. Always use common sense first in guessing what
a symbol means in a given context.
-
- A dot might indicate
- A dot product between vectors, if in between them.
- A time derivative of a quantity, if on top of it.
And also many more prosaic things (punctuation signs, decimal points,
...).
- Multiplication symbol. May indicate:
- An emphatic multiplication.
- Multiplication continued on the next line or from the previous
line.
- A vectorial product between vectors. In index notation,
the
-th component of
equals
where
is the index following
in the sequence
123123..., and
the one preceding it (or second
following). Alternatively, evaluate the determinant
- Might be used to indicate a factorial. Example:
5!
1
2
3
4
5
120.
The function that generalizes
to noninteger values of
is
called the gamma function;
. The
gamma function generalization is due to, who else, Euler. (However,
the fact that
instead of
is due to the idiocy of Legendre.) In
Legendre-resistant notation,
Straightforward integration shows that 0! is 1 as it should, and
integration by parts shows that
,
which ensures that the integral also produces the correct value of
for any higher integer value of
than 0. The integral,
however, exists for any real value of
above
1, not just
integers. The values of the integral are always positive, tending to
positive infinity for both
, (because the
integral then blows up at small values of
), and for
, (because the integral then blows up at
medium-large values of
). In particular, Stirling’s formula
says that for large positive
,
can be approximated as
where the value indicated by the dots becomes negligibly small for
large
. The function
can be extended further to any
complex value of
, except the negative integer values of
, where
is infinite, but is then no longer positive.
Euler’s integral can be done for
by making
the change of variables
, producing the
integral
, or
, which equals
and the integral under the square root can be done analytically
using polar coordinates. The result is that
To get
, multiply by
, since
.
A double exclamation mark may mean every second item is skipped,
e.g. 5!!
1
3
5. In general,
/
. Of course, 5!! should logically
mean (5!)!. Logic would indicate that 5
3
1
should be indicated by something like 5!’. But what is logic
in physics?
- May indicate:
- The magnitude or absolute value of the number or vector, if
enclosed between a pair of them.
- The determinant of a matrix, if enclosed between a pair of
them.
- The norm of the function, if enclosed between two pairs of them.
- Summation symbol. Example: if in three
dimensional space a vector
has components
2,
1,
4, then
stands
for
7.
One important thing to remember: the symbol used for the summation
index does not make a difference:
is
exactly the same as
. So freely
rename the index, but always make sure that the new name is not
already used for something else in the part that it appears in. If
you use the same name for two different things, it becomes a mess.
Related to that,
is not something
that depends on an index
. It is just a combined simple
number. Like 7 in the example above. It is commonly said that the
summation index sums away.
- Multiplication symbol. Example: if in three
dimensional space a vector
has components
2,
1,
4, then
stands
for
6.
One important thing to remember: the symbol used for the
multiplications index does not make a difference:
is exactly the same as
. So freely rename the index, but
always make sure that the new name is not already used for something
else in the part that it appears in. If you use the same name for
two different things, it becomes a mess.
Related to that,
is not something
that depends on an index
. It is just a combined simple
number. Like 6 in the example above. It is commonly said that the
multiplication index factors away.
(By who?)
- Integration symbol, the continuous version of
the summation symbol. For example,
is the summation of
over all infinitesimally small
fragments
that make up the entire
-range. For example,
equals 3
2
6; the average value of
between
0 and
2 is 3, and the sum of all the
infinitesimally small segments
gives the total length
2 of the range in
from 0 to 2.
One important thing to remember: the symbol used for the integration
variable does not make a difference:
is exactly the same as
. So
freely rename the integration variable, but always make sure that
the new name is not already used for something else in the part it
appears in. If you use the same name for two different things, it
becomes a mess.
Related to that
is not
something that depends on a variable
. It is just a
combined number. Like 6 in the example above. It is commonly said
that the integration variable integrates away.
- May indicate:
- An approaching process.
indicates
for practical purposes the value of the expression following the
when
is extremely small. Similarly,
indicates the value of the following
expression when
is extremely large.
- The fact that the left side leads to, or implies, the right-hand
side.
- Vector symbol. An arrow above a
letter indicates it is a vector. A vector is a quantity that
requires more than one number to be characterized. Typical vectors
in physics include position
, velocity
, linear
momentum
, acceleration
, force
, angular
momentum
, etcetera.
- May indicate:
- A derivative of a function. Examples:
0,
1,
,
,
.
- A small or modified quantity.
- The spatial differentiation operator
nabla. In Cartesian coordinates:
Nabla can be applied to a scalar function
in which case it gives
a vector of partial derivatives called the gradient of the
function:
Nabla can be applied to a vector in a dot product multiplication, in
which case it gives a scalar function called the divergence of the
vector:
or in index notation
Nabla can also be applied to a vector in a vectorial product
multiplication, in which case it gives a vector function called the curl or rot of the
vector. In index notation, the
-th component of this vector is
where
is the index following
in the sequence 123123...,
and
the one preceding it (or the second following it).
The operator
is called the Laplacian. In Cartesian coordinates:
Sometimes the Laplacian is indicated as
.
In non Cartesian coordinates, don’t guess; look these operators
up in a table book, [4, pp. 124-126]: . For example,
in spherical coordinates,
 |
(N.1) |
That allows the gradient of a scalar function
,
i.e.
, to be found immediately. But if you apply
on a vector, you have to be very careful because you also
need to differentiate
,
, and
. In particular, the correct divergence of a
vector
is
 |
(N.2) |
The curl
of the vector is
 |
(N.3) |
Finally the Laplacian is:
 |
(N.4) |
See also spherical coordinates.
Cylindrical coordinates are usually indicated as
,
and
. Here
is the Cartesian coordinate, while
is
the distance from the
-axis and
the angle around the
axis. In two dimensions, i.e. without the
terms, they are
usually called polar coordinates. In cylindrical coordinates:
 |
(N.5) |
 |
(N.6) |
 |
(N.7) |
 |
(N.8) |
- The
D'Alembertian is defined as
where
is a constant called the wave speed.
- A superscript star normally indicates a complex
conjugate. In the complex conjugate of a number, every
is
changed into a
.
- Less than.
- Less than or equal.
- Greater than.
- Greater than or equal.
- Equals sign. The quantity to the left is the same as
the one to the right.
- Emphatic equals sign. Typically means “by
definition equal” or
everywhere equal.
- Indicates approximately equal. Read it as “is
approximately equal to.”
- Indicates approximately equal. Often used when the
approximation applies only when something is small or large. Read
it as
is approximately equal to
or as “is
asymptotically equal to.”
- Proportional to. The two sides are equal
except for some unknown constant factor.
- (Gamma) May indicate:
- The Gamma function. Look under
!
for details.
- (capital delta) May indicate:
- An increment in the quantity following it.
- Often used to indicate the Laplacian
.
- (delta) May indicate:
- (partial) Indicates a vanishingly small
change or interval of the following variable. For example,
/
is the ratio of a vanishingly small
change in function
divided by the vanishingly small change in
variable
that causes this change in
. Such ratios define
derivatives, in this case the partial derivative of
with respect
to
.
- (variant of epsilon) May indicate:
- (eta) May be used to indicate a
-position.
- (capital theta) Used in this book to
indicate some function of
to be determined.
- (theta) May indicate:
- In spherical coordinates, the angle from the chosen
axis, with
apex at the origin.
- A
-position.
- A generic angle, like the one between the vectors in a
cross or dot product.
- (variant of theta) An alternate symbol
for
.
- (lambda) May indicate:
- Wave length.
- An eigenvalue.
- Some multiple of something.
- (xi) May indicate:
- An
-position.
- An integration variable.
- (pi) May indicate:
- A geometrical constant with value
3.141,592,653,589,793,238,462... The area of a circle
of radius
is
and its perimeter is
.
The volume of a sphere of radius
is
and its
surface is
. A 180
angle expressed
in radians is
. Note also that
1 and
1.
- (rho) May indicate:
- Scaled radial coordinate.
- Radial coordinate.
- (tau) May indicate:
- (capital phi) May indicate:
- Some function of
to be determined.
- (phi) May indicate:
- In spherical coordinates, the angle around the chosen
axis. Increasing
by
encircles the
-axis exactly
once.
- A phase angle.
- Something equivalent to an angle.
- (variant of phi) May indicate:
- A change in angle
.
- An alternate symbol for
.
- (omega) May indicate:
- May indicate:
- Some generic matrix.
- Some constant.
- Area.
- May indicate:
- Acceleration.
- Start point of an integration interval.
- Some coefficient.
- Some constant.
- absolute
- May indicate:
- The absolute value of a real number
is indicated by
.
It equals
is
is positive or zero and
if
is
negative.
- The absolute value of a complex number
is indicated by
. It equals the length of the number plotted as a vector in
the complex plane. This simplifies to above definition if
is
real.
- adjoint
- The adjoint
or
of a matrix is the
complex-conjugate transpose of the matrix.
Alternatively, it is the matrix you get if you take it to the other
side of an inner product. (While keeping the value of the inner
product the same regardless of whatever two vectors or functions may
be involved.)
Hermitian
matrices are self-adjoint;
they are equal to their adjoint. Skew-Hermitian
matrices are the negative of their adjoint.
Unitary
matrices are the inverse of their adjoint. Unitary matrices
generalize rotations and reflections of vectors. Unitary operators
preserve inner products.
Fourier transforms are unitary operators on account of the Parseval
equality that says that inner products are preserved.
- angle
- Consider two semi-infinite lines extending from a
common intersection point. Then the angle between these lines is
defined in the following way: draw a unit circle in the plane of the
lines and centered at their intersection point. The angle is then
the length of the circular arc that is in between the lines. More
precisely, this gives the angle in radians, rad. Sometimes an angle
is expressed in degrees, where
rad is taken to be
360
. However, using degrees is usually a very bad
idea in science.
In three dimensions, you may be interested in the so-called
solid angle
inside a conical surface. This
angle is defined in the following way: draw a sphere of unit radius
centered at the apex of the conical surface. Then the solid angle
is the area of the spherical surface that is inside the cone. Solid
angles are in steradians. The cone does not need to be a circular
one, (i.e. have a circular cross section), for this to apply. In
fact, the most common case is the solid angle corresponding to an
infinitesimal element
of spherical
coordinate system angles. In that case the surface of the unit
sphere inside the conical surface is is approximately rectangular,
with sides
and
. That makes
the enclosed solid angle equal to
.
- May indicate:
- A generic second matrix.
- Some constant.
- May indicate:
- End point of an integration interval.
- Some coefficient.
- Some constant.
- basis
- A basis is a minimal set of vectors or functions that
you can write all other vectors or functions in terms of. For
example, the unit vectors
,
, and
are a basis
for normal three-dimensional space. Every three-dimensional vector
can be written as a linear combination of the three.
- May indicate:
- A third matrix.
- A constant.
- Cauchy-Schwartz inequality
- The Cauchy-Schwartz inequality
describes a limitation on the magnitude of inner products. In
particular, it says that for any vectors
and
For example, if
and
are real vectors, the inner
product is the dot product and we have
where
is the length of vector
and
the
one of
, and
is the angle in between the two
vectors. Since a cosine is less than one in magnitude, the
Cauchy-Schwartz inequality is therefore true for vectors.
- The cosine function, a periodic function
oscillating between 1 and -1 as shown in
[4, pp. 40-]. See also
sin.
- curl
- The curl of a vector
is defined as
.
- Indicates a vanishingly small change or
interval of the following variable. For example,
can be
thought of as a small segment of the
-axis.
In three dimensions,
is an
infinitesimal volume element. The symbol
means that you sum
over all such infinitesimal volume elements.
- derivative
- A derivative of a function is the ratio of a
vanishingly small change in a function divided by the vanishingly
small change in the independent variable that causes the change in
the function. The derivative of
with respect to
is
written as
/
, or also simply as
. Note that the
derivative of function
is again a function of
: a ratio
can be found at every point
. The derivative of a function
with respect to
is written as
/
to indicate that there are other
variables,
and
, that do not vary.
- determinant
- The determinant of a square matrix
is a
single number indicated by
. If this number is nonzero,
can be any vector
for the right choice of
. Conversely, if the determinant is zero,
can only produce a very limited set of vectors, though if it can
produce a vector
, it can do so for multiple vectors
.
There is a recursive algorithm that allows you to compute
determinants from increasingly bigger matrices in terms of
determinants of smaller matrices. For a 1
1 matrix
consisting of a single number, the determinant is simply that
number:
(This determinant should not be confused with the absolute value of
the number, which is written the same way. Since you normally do
not deal with 1
1 matrices, there is normally no
confusion.) For 2
2 matrices, the determinant can be
written in terms of 1
1 determinants:
so the determinant is
in short.
For 3
3 matrices, you have
and you already know how to work out those 2
2 determinants,
so you now know how to do 3
3 determinants. Written out fully:
For 4
4 determinants,
Etcetera. Note the alternating sign pattern of the terms.
As you might infer from the above, computing a good size determinant
takes a large amount of work. Fortunately, it is possible to
simplify the matrix to put zeros in suitable locations, and that can
cut down the work of finding the determinant greatly. You are
allowed to use the following manipulations without seriously
affecting the computed determinant:
- You can
transpose
the matrix, i.e. change its columns into its rows.
- You can create zeros in a row by subtracting a suitable
multiple of another row.
- You can also swap rows, as long as you remember that each time
that you swap two rows, it will flip over the sign of the computed
determinant.
- You can also multiply an entire row by a constant, but that will
multiply the computed determinant by the same constant.
Applying these tricks in a systematic way, called “Gaussian
elimination” or “reduction to lower triangular
form”, you can eliminate all matrix coefficients
for
which
is greater than
, and that makes evaluating the
determinant pretty much trivial.
- div(ergence)
- The divergence of a vector
is defined
as
.
- May indicate:
- The basis for the natural logarithms. Equal to
2.718,281,828,459... This number produces the
exponential function
, or
, or in words
to the power 
,
whose derivative with respect to
is again
. If
is a constant, then the derivative of
is
.
Also, if
is an ordinary real number, then
is a
complex number with magnitude 1.
- Assuming that
is an ordinary real
number, and
a real variable,
is a complex function
of magnitude one. The derivative of
with respect to
is
- eigenvector
- A concept from linear algebra. A vector
is an eigenvector of a matrix
if
is nonzero
and
for some number
called the corresponding eigenvalue.
- exponential function
- A function of the form
,
also written as
. See function
and
.
- May indicate:
- The anti-derivative of some function
.
- Some function.
- May indicate:
- A generic function.
- A fraction.
- Frequency.
- function
- A mathematical object that associates values with
other values. A function
associates every value of
with
a value
. For example, the function
associates
0 with
0,
with
,
1 with
1,
2 with
4,
3 with
9, and more generally, any
arbitrary value of
with the square of that value
.
Similarly, function
associates any arbitrary
with its cube
,
associates any arbitrary
with the sine of that value, etcetera.
One way of thinking of a function is as a procedure that allows you,
whenever given a number, to compute another number.
- functional
- A functional associates entire functions with
single numbers. For example, the expectation energy is
mathematically a functional: it associates any arbitrary wave
function with a number: the value of the expectation energy if
physics is described by that wave function.
- May indicate:
- A second generic function.
- Gauss' Theorem
- This theorem, also called divergence theorem
or Gauss-Ostrogradsky theorem, says that for a continuously
differentiable vector
,
where the first integral is over the volume of an arbitrary region
and the second integral is over all the surface area of that region;
is at each point found as the unit vector that is normal
to the surface at that point.
- grad(ient)
- The gradient of a scalar
is defined as
.
- hypersphere
- A hypersphere is the generalization of the
normal three-dimensional sphere to
-dimensional space. A sphere of
radius
in three-dimensional space consists of all points
satisfying
where
,
, and
are Cartesian coordinates
with origin at the center of the sphere. Similarly a hypersphere in
-dimensional space is defined as all points satisfying
So a two-dimensional hypersphere
of radius
is really
just a circle of radius
. A one-dimensional
hypersphere
is really just the line segment 

.
The volume”
and surface “area
of an
-dimensional hypersphere is given by
(This is readily derived recursively. For a sphere of unit radius,
note that the
-dimensional volume
is an integration of
-dimensional volumes with respect to
. Then renotate
as
and look up the resulting integral in a table book.
The formula for the area follows because
where
is the distance from the origin.) In three dimensions,
according to the above formula. That makes the
three-dimensional volume

3 equal to the
actual volume of the sphere, and the three-dimensional
area
equal to the actual surface area.
On the other hand in two dimensions,
. That makes the
two-dimensional volume
really the area
of the circle. Similarly the two-dimensional surface
area
is really the perimeter of the
circle. In one dimensions
and the volume
is really the length of the interval, and the
area
2 is really its number of end points.
Often the infinitesimal
-dimensional volume
element
is needed. This is the infinitesimal integration
element for integration over all coordinates. It is:
Specifically, in two dimensions:
while in three dimensions:
The expressions in parentheses are
in polar coordinates,
respectively
in spherical coordinates.
- The imaginary part of a complex number. If
with
and
real numbers, then
. Note that
.
- May indicate:
- The number of a particle.
- A summation index.
- A generic index or counter.
Not to be confused with
.
- The unit vector in the
-direction.
- The standard square root of minus one:
,
1, 1/
,
.
- index notation
- A more concise and powerful way of writing
vector and matrix components by using a numerical index to indicate
the components. For Cartesian coordinates, you might number the
coordinates
as 1,
as 2, and
as 3. In that case, a sum
like
can be more concisely written as
.
And a statement like
0,
0,
0 can
be more compactly written as
0. To really see how it
simplifies the notations, have a look at the matrix entry. (And that
one shows only 2 by 2 matrices. Just imagine 100 by 100 matrices.)
- iff
- Emphatic
if.
Should be read as
if and only if.
- integer
- Integer numbers are the whole numbers:
.
- inverse
- (Of matrices.) If a matrix
converts a vector
into a vector
, then the inverse of the matrix,
, converts
back into
.
In other words,
with
the unit, or
identity, matrix.
The inverse of a matrix only exists if the matrix is square and has
nonzero determinant.
- irrotational
- A vector
is irrotational if its curl
is zero.
- May indicate:
- A summation index.
- A generic index or counter.
- The unit vector in the
-direction.
- May indicate:
- A generic summation index.
- The unit vector in the
-direction.
- May indicate:
- A generic summation index.
- May indicate:
- Indicates the final result of an approaching
process.
indicates for practical purposes
the value of the following expression when
is
extremely small.
- linear combination
- A very generic concept indicating sums of
objects times coefficients. For example, a position vector
is
the linear combination
with the objects the
unit vectors
,
, and
and the coefficients the
position coordinates
,
, and
. A linear combination of a
set of functions
would be the
function
where
are constants, i.e. independent of
.
- linear dependence
- A set of vectors or functions is linearly
dependent if at least one of the set can be expressed in terms of
the others. Consider the example of a set of functions
. This set is linearly dependent
if
where at least one of the constants
is
nonzero. To see why, suppose that say
is nonzero. Then you
can divide by
and rearrange to get
That expresses
in terms of the other functions.
- linear independence
- A set of vectors or functions is
linearly independent if none of the set can be expressed in terms of
the others. Consider the example of a set of functions
. This set is linearly
independent if
only if every one of the constants
is zero.
To see why, assume that say
could be expressed in terms
of the others,
Then taking
1,
,
, ...
, the condition above
would be violated. So
cannot be expressed in terms of the
others.
- May indicate:
- Number of rows in a matrix.
- A generic summation index or generic integer.
- matrix
- A table of numbers.
As a simple example, a two-dimensional matrix
is a table of four
numbers called
,
,
, and
:
unlike a two-dimensional (ket) vector
, which would consist
of only two numbers
and
arranged in a column:
(Such a vector can be seen as a rectangular matrix
of size 2
1, but let’s not get into that.)
In index notation, a matrix
is a set of numbers
indexed by two indices. The first index
is the row number, the
second index
is the column number. A matrix turns a vector
into another vector
according to the recipe
where
stands for “the
-th component of vector
,” and
for “the
-th component of
vector
.”
As an example, the product of
and
above is by
definition
which is another two-dimensional ket vector.
Note that in matrix multiplications like the example above, in
geometric terms you take dot products between the rows of the first
factor and the column of the second factor.
To multiply two matrices together, just think of the columns of the
second matrix as separate vectors. For example:
which is another two-dimensional matrix. In index notation, the
component of the product matrix has value
.
The zero matrix is like the number
zero; it does not change a matrix it is added to and turns
whatever it is multiplied with into zero. A zero matrix is zero
everywhere. In two dimensions:
A unit matrix is the equivalent of the
number one for matrices; it does not change the quantity it is
multiplied with. A unit matrix is one on its “main
diagonal” and zero elsewhere. The 2 by 2 unit matrix is:
More generally the coefficients,
, of a unit matrix
are one if
and zero otherwise.
The transpose of a matrix
,
, is what you get if you
switch the two indices. Graphically, it turns its rows into its
columns and vice versa. The Hermitian adjoint
is what you get if you switch the two indices and then take
the complex conjugate of every element. If you want to take a
matrix to the other side of an inner product, you will need to
change it to its Hermitian adjoint. Hermitian matrices
are equal to their Hermitian adjoint, so this does nothing for them.
See also determinant
and
eigenvector.
- May indicate:
- Number of columns in a matrix.
- A generic summation index or generic integer.
- A natural number.
and maybe some other stuff.
- natural
- Natural numbers are the numbers:
.
- normal
- A normal operator or matrix is one that has
orthonormal eigenfunctions or eigenvectors. Since eigenvectors are
not orthonormal in general, a normal operator or matrix is abnormal!
For an operator or matrix
to be normal,
it must
commute with its Hermitian adjoint,
0. Hermitian
matrices are normal since they are equal to their Hermitian adjoint.
Skew-Hermitian matrices are normal since they are equal to the
negative of their Hermitian adjoint. Unitary matrices are normal
because they are the inverse of their Hermitian adjoint.
- O
- May indicate the origin of the coordinate system.
- opposite
- The opposite of a number
is
. In other
words, it is the additive inverse.
- perpendicular bisector
- For two given points
and
, the
perpendicular bisector consists of all points
that are equally
far from
as they are from
. In two dimensions, the
perpendicular bisector is the line that passes through the point
exactly half way in between
and
, and that is orthogonal to
the line connecting
and
. In three dimensions, the
perpendicular bisector is the plane that passes through the point
exactly half way in between
and
, and that is orthogonal to
the line connecting
and
. In vector notation, the
perpendicular bisector of points
and
is all points
whose
radius vector
satisfies the equation:
(Note that the halfway point
is included in this formula, as is the half
way point plus any vector that is normal to
.)
- phase angle
- Any complex number can be written in
polar form
as
where
both the magnitude
and the phase angle
are real
numbers. Note that when the phase angle varies from zero to
,
the complex number
varies from positive real to positive
imaginary to negative real to negative imaginary and back to
positive real. When the complex number is plotted in the complex
plane, the phase angle is the direction of the number relative to
the origin. The phase angle
is often called the argument,
but so is about everything else in mathematics, so that is not very
helpful.
In complex time-dependent waves of the form
, and its real equivalent
, the phase angle
gives the
angular argument of the wave at time zero.
- May indicate:
- Charge.
- Heat flux density.
- May indicate:
- Some radius.
- Some function of
to be determined.
- The real part of a complex number. If
with
and
real numbers, then
. Note that
.
- May indicate:
- The radial distance from the chosen origin of the coordinate
system.
typically indicates the
-th Cartesian component of
the radius vector
.
- Some ratio.
- The position vector. In Cartesian coordinates
or
. In spherical coordinates
. Its three Cartesian components may be indicated by
or by
or by
.
- reciprocal
- The reciprocal of a number
is 1/
.
In other words, it is the multiplicative inverse.
- rot
- The rot of a vector
is defined as
.
- scalar
- A quantity characterized by a single number.
- The sine function, a periodic function
oscillating between 1 and -1 as shown in
[4, pp. 40-]. Good to remember:
1 and
and
.
- spherical coordinates
- The spherical coordinates
,
, and
of an arbitrary point P are defined as
Figure N.1:
Spherical coordinates of an arbitrary point P.
 |
In Cartesian coordinates, the unit vectors in the
,
, and
directions are called
,
, and
. Similarly, in spherical
coordinates, the unit vectors in the
,
, and
directions are called
,
,
and
. Here, say, the
direction is
defined as the direction of the change in position if you increase
by an infinitesimally small amount while keeping
and
the same. Note therefore in particular that the direction
of
is the same as that of
; radially outward.
An arbitrary vector
can be decomposed in components
,
, and
along these unit
vectors. In particular
Recall from calculus that in spherical coordinates, a volume
integral of an arbitrary function
takes the form
In other words, the volume element in spherical coordinates is
Often it is convenient of think of volume integrations as a two-step
process: first perform an integration over the angular coordinates
and
. Physically, that integrates over
spherical surfaces. Then perform an integration over
to
integrate all the spherical surfaces together. The combined
infinitesimal angular integration element
is called the infinitesimal solid angle
. In two-dimensional polar coordinates
and
, the equivalent would be the infinitesimal polar
angle
. Recall that
, (in proper
radians of course), equals the arclength of an infinitesimal part of
the circle of integration divided by the circle radius. Similarly
is the surface of an infinitesimal part of the sphere of
integration divided by the square sphere radius.
See the 
entry for the gradient operator
and Laplacian in spherical coordinates.
- Stokes' Theorem
- This theorem, first derived by Kelvin and
first published by someone else I cannot recall, says that for any
reasonably smoothly varying vector
,
where the first integral is over any smooth surface area
and the
second integral is over the edge of that surface. How did Stokes
get his name on it? He tortured his students with it, that’s
how!
One important consequence of the Stokes theorem is for vector fields
that are irrotational,
i.e. that have
0. Such fields can be written as
Here
is the position of an arbitrarily chosen
reference point, usually the origin. The reason the field
can be written this way is the Stokes theorem. Because of the
theorem, it does not make a difference along which path from
to
you integrate. (Any two paths give the
same answer, as long as
is irrotational everywhere in
between the paths.) So the definition of
is unambiguous. And
you can verify that the partial derivatives of
give the
components of
by approaching the final position
in
the integration from the corresponding direction.
- symmetry
- A symmetry is an operation under which an object
does not change. For example, a human face is almost, but not
completely, mirror symmetric: it looks almost the same in a mirror
as when seen directly. The electrical field of a single point
charge is spherically symmetric; it looks the same from whatever
angle you look at it, just like a sphere does. A simple smooth
glass (like a glass of water) is cylindrically symmetric; it looks
the same whatever way you rotate it around its vertical axis.
- May indicate:
- triple product
- A product of three vectors. There are two
different versions:
- The scalar triple product
. In index notation,
where
is the index following
in the sequence
123123..., and
the one preceding it. This triple
product equals the determinant
formed
with the three vectors. Geometrically, it is plus or minus the
volume of the parallelepiped that has vectors
,
, and
as edges. Either way, as long as the
vectors are normal vectors and not operators,
and you can change the two sides of the dot product without
changing the triple product, and/or you can change the sides in
the vectorial product with a change of sign.
- The vectorial triple product
. In index notation, component number
of this triple product is
which may be rewritten as
In particular, as long as the vectors are normal ones,
- May indicate:
- The first velocity component in a Cartesian coordinate system.
- An integration variable.
- May indicate:
- Volume.
Volume
in
-dimensions (i.e. line segment
length in one dimensions, area in two, volume in three, etc.)
- May indicate:
- The second velocity component in a Cartesian coordinate system.
- Magnitude of a velocity (speed).
- May indicate:
- Velocity vector.
- Generic vector.
- vector
- A quantity characterized by a list of numbers. A
vector
in index notation is a set of numbers
indexed by an index
. In normal three-dimensional Cartesian
space,
takes the values 1, 2, and 3, making the vector a list of
three numbers,
,
, and
. These numbers are called
the three components of
.
- vectorial product
- An vectorial product, or cross product is a product of vectors
that produces another vector. If
it means in index notation
that the
-th component of vector
is
where
is the index following
in the sequence 123123...,
and
the one preceding it. For example,
will equal
.
- May indicate:
- The third velocity component in a Cartesian coordinate system.
- Weight factor.
- Generic vector.
- Used in this book to indicate a function of
to be determined.
- May indicate:
- First coordinate in a Cartesian coordinate system.
- A generic argument of a function.
- An unknown value.
- Used in this book to indicate a function of
to be determined.
- May indicate:
- Second coordinate in a Cartesian coordinate system.
- A second generic argument of a function.
- A second unknown value.
- Used in this book to indicate a function of
to be determined.
- May indicate:
- Third coordinate in a Cartesian coordinate system.
- A third generic argument of a function.
- A third unknown value.