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 A380091 #16 Feb 03 2025 23:05:18 %S A380091 2,3,7,31,991,1951,2521,7411,23431,26731,37441,92431,131071,396631, %T A380091 489061,532141,830551,2811691,3319171,3698941,4247167,5239411,6829681, %U A380091 8326711,8997871,12625831,12889231,14756743,15891121,16125721,16446301,21203071 %N A380091 Primes p such that phi(p+1) = 2*phi(p-1) where phi = A000010. %C A380091 Primes in A258454 - 1. %t A380091 Select[Prime[Range[10^5]], EulerPhi[# + 1] == 2*EulerPhi[# - 1] &] (* _Amiram Eldar_, Jan 12 2025 *) %o A380091 (Magma) [p: p in PrimesUpTo(15*10^6) | 2*EulerPhi(p-1) eq EulerPhi(p+1)]; %Y A380091 Cf. A000010, A066812, A067890, A258454. %K A380091 nonn %O A380091 1,1 %A A380091 _Juri-Stepan Gerasimov_, Jan 11 2025