Theorem: For all integers k, if k > 0 then k2 + 2k + 1 is composite.
“Proof: Suppose k is any integer such that k > 0. If
k2 + 2k + 1 is composite, then k2 + 2k + 1 = rs for some
integers r and s such that
1 < r < (k2 + 2k + 1)
and 1 < s < (k2 + 2k + 1).
Since k2 + 2k + 1 = rs
and both r and s are strictly between 1 and k2 + 2k + 1,then k2 + 2k + 1 is not prime. Hence k2 + 2k + 1 is composite
as was to be shown.”