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 A065875 #16 Dec 15 2024 07:26:20 %S A065875 3,4,5,6,8,10,12,17,32,34,40,48,60,257,512,514,544,640,680,768,816, %T A065875 960,1020,65537,131072,131074,131584,139264,139808,163840,164480, %U A065875 174080,174760,196608,197376,208896,209712,245760,246720,261120,262140 %N A065875 Numbers k such that usigma(phi(k)) is a prime. %C A065875 The only odd terms below 10^7 are 3, 5, 17, 257 and 65537. %C A065875 Numbers k such that phi(k) = 2^(2^m) where 2^(2^m)+1 is a Fermat prime (A019434). a(42) >= 2^(2^33) + 1, if a 6th Fermat prime exists. - _Amiram Eldar_, Dec 14 2024 %o A065875 (PARI) u(n)=sumdivmult(n, d, if(gcd(d, n/d)==1, d)); %o A065875 for(n=1,10^5,if(isprime(u(eulerphi(n))),print1(n,", "))); \\ _Joerg Arndt_, Sep 17 2023 %Y A065875 Cf. A062514, A034448, A000010, A019434. %K A065875 nonn %O A065875 1,1 %A A065875 _Robert G. Wilson v_, Dec 07 2001 %E A065875 Deleted incorrect MMA program. - _N. J. A. Sloane_, Sep 17 2023