2.42.24 Build a matrix with a function : makemat
makemat takes three arguments :

a function of two variables j and k which
should return the value of a_{j,k}, the element of
row index j and column index k of the matrix to be built.
 two integers n and p.
makemat returns the matrix A=(a_{j,k})
(j=0..n−1 and k=0..p−1) of dimension n × p.
Input :
makemat((j,k)>j+k,4,3)
or first define the h function:
h(j,k):=j+k
then, input:
makemat(h,4,3)
Output :
[[0,1,2],[1,2,3],[2,3,4],[3,4,5]]
Note that the indices are counted starting from 0.