May 5
{m=p+1 {n=q+1 {k=mn-(m+n) = pq-1 {f(mn) = k f((p + 1)(q + 1)) = pq-1
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{m=p+1 {n=q+1 {k=mn-(m+n) = pq-1 {f(mn) = k f((p + 1)(q + 1)) = pq-1