This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A216419 #24 Feb 16 2025 08:33:18
%S A216419 225,441,1089,1225,1521,2025,2601,3025,3249,3375,3969,4225,4761,5625,
%T A216419 5929,7225,7569,8281,8649,9025,9261,9801,11025,12321,13225,13689,
%U A216419 14161,15129,16641,17689,18225,19881,20449,21025,21609,23409,24025,25281,25921,27225
%N A216419 Odd powers that are not prime powers.
%C A216419 Numbers in A075109 but not in A000961.
%C A216419 Also odd perfect powers having no primitive root (intersection of A075109 and A175594).
%H A216419 Arkadiusz Wesolowski, <a href="/A216419/b216419.txt">Table of n, a(n) for n = 1..10000</a>
%H A216419 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PrimitiveRoot.html">Primitive Root</a>.
%F A216419 Sum_{n>=1} 1/a(n) = 1/2 + Sum_{k>=2} mu(k)*(1-zeta(k)*(2^k-1)/2^k) - Sum_{p prime} 1/(p*(p-1)) = 0.0158808884... - _Amiram Eldar_, Dec 21 2020
%e A216419 81 = 9^2 as well as 81 = 3^4, therefore 81 is not a term.
%e A216419 225 can be expressed so in one way as (3*5)^2, therefore 225 is a term.
%t A216419 nn = 27500; lst = Union[Flatten[Table[n^i, {i, Prime[Range[PrimePi[Log[2, nn]]]]}, {n, 2, nn^(1/i)}]]]; Select[lst, OddQ[#] && ! IntegerQ@PrimitiveRoot[#] &] (* Most of the code is from _T. D. Noe_ *)
%o A216419 (Magma) [n : n in [3..27225 by 2] | IsPower(n) and EulerPhi(n) ne CarmichaelLambda(n)]; // _Arkadiusz Wesolowski_, Nov 09 2013
%Y A216419 Cf. A008683, A075109, A136141, A175594.
%K A216419 nonn
%O A216419 1,1
%A A216419 _Arkadiusz Wesolowski_, Sep 06 2012