In the usual case of \((m,n)=1\), returns a square matrix
whose rows are a
and c(x,y)
. This matrix is a unimodular
transformation that takes a pair of basic periods to another pair of
basic periods.
If \((m,n)\neq 1\) then more than one solution is
available (for example congruence(c(4,6),2)
). In this case, extra rows
are added and the matrix is no longer square.