Main Content

Simplify or convert Galois field element formatting

`tp = gftuple(a,m)`

tp = gftuple(a,prim_poly)

tp = gftuple(a,m,p)

tp = gftuple(a,prim_poly,p)

tp = gftuple(a,prim_poly,p,prim_ck)

[tp,expform] = gftuple(...)

**Note**

This function performs computations in GF(p^{m}),
where p is prime. To perform equivalent computations in GF(2^{m}),
apply the `.^`

operator and the `log`

function
to Galois arrays. For more information, see Example: Exponentiation and Example: Elementwise Logarithm.

`gftuple`

serves to simplify the polynomial
or exponential format of Galois field elements, or to convert from
one format to another. For an explanation of the formats that `gftuple`

uses,
see Representing Elements of Galois Fields.

In this discussion, the format of an element of GF(p^{m})
is called “simplest” if all exponents of the primitive
element are

Between 0 and m-1 for the polynomial format

Either

`-Inf`

, or between 0 and p^{m-2}, for the exponential format

For all syntaxes, `a`

is a matrix, each row
of which represents an element of a Galois field. The format of `a`

determines
how MATLAB interprets it:

If

`a`

is a column of integers, MATLAB interprets each row as an*exponential*format of an element. Negative integers are equivalent to`-Inf`

in that they all represent the zero element of the field.If

`a`

has more than one column, MATLAB interprets each row as a*polynomial*format of an element. (Each entry of`a`

must be an integer between 0 and`p`

-1.)

The exponential or polynomial formats mentioned above are all
relative to a primitive element specified by the *second* input
argument. The second argument is described below.

`tp = gftuple(a,m)`

returns
the simplest polynomial format of the elements that `a`

represents,
where the kth row of `tp`

corresponds to the kth
row of `a`

. The formats are relative to a root of
the default primitive polynomial for GF(`2^m`

), where `m`

is
a positive integer.

`tp = gftuple(a,prim_poly)`

is
the same as the syntax above, except that `prim_poly`

is
a polynomial character vector or
a row vector that lists the coefficients of a degree `m`

primitive
polynomial for GF(`2^m`

) in order of ascending exponents.

`tp = gftuple(a,m,p)`

is the same as `tp = gftuple(a,m)`

except that 2
is replaced by a prime number `p`

.

`tp = gftuple(a,prim_poly,p)`

is
the same as `tp = gftuple(a,prim_poly)`

except that
2 is replaced by a prime number `p`

.

`tp = gftuple(a,prim_poly,p,prim_ck)`

is
the same as `tp = gftuple(a,prim_poly,p)`

except
that `gftuple`

checks whether `prim_poly`

represents
a polynomial that is indeed primitive. If not, then `gftuple`

generates
an error and `tp`

is not returned. The input argument `prim_ck`

can
be any number or character vector; only its existence matters.

`[tp,expform] = gftuple(...)`

returns
the additional matrix `expform`

. The kth row of `expform`

is
the simplest exponential format of the element that the kth row of `a`

represents.
All other features are as described in earlier parts of this “Description”
section, depending on the input arguments.

As another example, the `gftuple`

command below
generates a list of elements of GF(`p^m`

), arranged
relative to a root of the default primitive polynomial. Some functions
in this toolbox use such a list as an input argument.

p = 5; % Or any prime number m = 4; % Or any positive integer field = gftuple([-1:p^m-2]',m,p);

Finally, the two commands below illustrate the influence of
the *shape* of the input matrix. In the first command,
a column vector is treated as a sequence of elements expressed in
exponential format. In the second command, a row vector is treated
as a single element expressed in polynomial format.

tp1 = gftuple([0; 1],3,3) tp2 = gftuple([0, 0, 0, 1],3,3)

The output is below.

tp1 = 1 0 0 0 1 0 tp2 = 2 1 0

The outputs reflect that, according to the default primitive
polynomial for GF(3^{3}), the relations below
are true.

$$\begin{array}{l}{\alpha}^{0}=1+0\alpha +0{\alpha}^{2}\\ {\alpha}^{1}=0+1\alpha +0{\alpha}^{2}\\ 0+0\alpha +0{\alpha}^{2}+{\alpha}^{3}=2+\alpha +0{\alpha}^{2}\end{array}$$

`gftuple`

uses recursive callbacks to determine
the exponential format.