%N Divisible exactly by the square of a prime.
%C Numbers for which at least one prime factor exponent is exactly 2.
%C Sometimes called squarefull numbers, although that term is usually reserved for A001694. - _N. J. A. Sloane_, Jul 22 2012
%H R. J. Mathar, <a href="/A038109/b038109.txt">Table of n, a(n) for n = 1..1247</a>
%e 20=5*2*2 is divisible by 2^2.
%p isA038109 := proc(n)
%p local p;
%p for p in ifactors(n) do
%p if op(2,p) = 2 then
%p return true;
%p end if;
%p end do:
%p false ;
%p end proc: # _R. J. Mathar_, Dec 08 2015
%t Select[Range,MemberQ[Transpose[FactorInteger[#]][],2]&] (* _Harvey P. Dale_, Sep 24 2012 *)
%o (PARI) is(n)=#select(n->n==2, Set(factor(n)[,2])) \\ _Charles R Greathouse IV_, Sep 17 2015
%Y Cf. A001694, A013929, A284017, A284018.
%A _Felice Russo_
%E Corrected and extended by _Erich Friedman_