Fermat (named after Pierre de Fermat) is a freeware program developed by Prof. Robert H. Lewis of Fordham University. It is a computer algebra system, in which items being computed can be integers (of arbitrary size), rational numbers, real numbers, complex numbers, modular numbers, finite field elements, multivariable polynomials, rational functions, or polynomials modulo other polynomials. The main areas of application are multivariate rational function arithmetic and matrix algebra over rings of multivariate polynomials or rational functions. Fermat does not do simplification of transcendental functions or symbolic integration. A session with Fermat usually starts by choosing rational or modular "mode" to establish the ground field (or ground ring) as or . On top of this may be attached any number of symbolic variables thereby creating the polynomial ring and its quotient field. Further, some polynomials involving some of the can be chosen to mod out with, creating the quotient ring Finally, it is possible to allow Laurent polynomials, those with negative as well as positive exponents. Once the computational ring is established in this way, all computations are of elements of this ring. The computational ring can be changed later in the session. The polynomial gcd procedures, which call each other in a highly recursive manner, are about 7000 lines of code. Fermat has extensive built-in primitives for array and matrix manipulations, such as submatrix, sparse matrix, determinant, normalize, column reduce, row echelon, Smith normal form, and matrix inverse. It is consistently faster than some well known computer algebra systems, especially in multivariate polynomial gcd. It is also space efficient. The basic data item in Fermat is a multivariate rational function or quolynomial. The numerator and denominator are polynomials with no common factor. Polynomials are implemented recursively as general linked lists, unlike some systems that implement polynomials as lists of monomials. To implement (most) finite fields, the user finds an irreducible monic polynomial in a symbolic variable, say and commands Fermat to mod out by it. This may be continued recursively, etc. Low level data structures are set up to facilitate arithmetic and gcd over this newly created ground field. Two special fields, and are more efficiently implemented at the bit level. History With Windows 10, and thanks to Bogdan Radu, it is now possible (May 2021) to run Fermat Linux natively on Windows. See the main web page http://home.bway.net/lewis Fermat was last updated on 20 May 2020 (Mac and