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 A252021 #24 Sep 08 2022 08:46:10 %S A252021 13,19,41,61,89,137,157,229,233,277,349,373,569,709,733,809,853,857, %T A252021 877,997,1049,1069,1097,1193,1213,1237,1433,1669,1789,1913,2153,2293, %U A252021 2389,2677,2749,2777,2797,3209,3229,3253,3373,3449,3517,3593,3733,3833,3929,4073,4457,4549,4597,4793,4813,4909,4937,5197,5273 %N A252021 Primes p such that (p - 1)/tau(p - 1) is also prime. %H A252021 Amiram Eldar, <a href="/A252021/b252021.txt">Table of n, a(n) for n = 1..10000</a> %e A252021 a(1) = 13, since 12/tau(12) = 2. %e A252021 a(2) = 19, since 18/tau(18) = 3. %e A252021 a(4) = 61, since 60/tau(60) = 5. %t A252021 Select[Prime[Range[1000]], PrimeQ[(# - 1)/DivisorSigma[0, # - 1]] &] %o A252021 (Magma) [p:p in PrimesUpTo(5300)| ((p-1) mod NumberOfDivisors(p-1) eq 0) and IsPrime((p-1) div NumberOfDivisors(p-1)) ]; // _Marius A. Burtea_, Dec 30 2019 %Y A252021 Cf. A000005, A008328, A033950. %K A252021 nonn,easy %O A252021 1,1 %A A252021 _Carlos Eduardo Olivieri_, Mar 21 2015