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 A113609 #12 Feb 16 2025 08:32:59
%S A113609 1,2,3,3,4,4,5,5,6,6,7,7,7,7,7,7,8,8,8,8,8,8,9,9,10,10,11,11,12,12,12,
%T A113609 12,12,12,12,12,12,12,12,12,13,13,13,13,13,13,14,14,14,14,14,14,14,14,
%U A113609 14,14,14,14,15,15,15,15,15,15,15,15,15,15,15,15,16,16,16,16,16,16,16
%N A113609 Number of prime powers q<=n such that also q+2 is a prime power.
%C A113609 a(n) > A071538(n);
%C A113609 (25,27) is the smallest pair of prime powers (q,q+2) such that both q and q+2 are not primes, conjecture: there are more (but not < 10^6).
%H A113609 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PrimePower.html">Prime Power</a>
%H A113609 <a href="/index/Pri#primepop">Index entries for sequences related to numbers of primes in various ranges</a>
%e A113609 a(10) = Card{(p^0,3), (2,2^2), (3,5), (5,7), (7,3^2), (3^2,11)} = 6.
%o A113609 (PARI) ispp(n) = (n==1) || isprimepower(n);
%o A113609 a(n) = sum(k=1, n, ispp(k) && ispp(k+2)); \\ _Michel Marcus_, Jun 24 2019
%Y A113609 Cf. A000961, A071538.
%K A113609 nonn
%O A113609 1,2
%A A113609 _Reinhard Zumkeller_, Jan 14 2006