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 A376338 #7 Oct 18 2024 18:13:53
%S A376338 11,19,43,49,67,163,211,283,331,523,547,691,787,907,1051,1123,1171,
%T A376338 1531,1723,1867,2011,2083,2251,2347,2371,2467,2707,2731,2803,2971,
%U A376338 3187,3307,3547,3643,3907,3931,4051,4243,4363,4603,4651,4723,5107,5227,5443,5923
%N A376338 Numbers k such that phi(k)/2 - 1 = phi(k + 1) where phi = A000010.
%C A376338 Conjecture: this sequence is the union {49} and the primes of the form 4*p - 1 where p odd prime.
%e A376338 Number 49 is in this sequence because phi(49)/2 - 1 = 42/2 - 1 = 21 - 1 = 20 is equal to phi(49 + 1) = phi(50) = 20.
%t A376338 Select[Range[6000], EulerPhi[#]/2-1==EulerPhi[#+1] &] (* _Stefano Spezia_, Sep 22 2024 *)
%o A376338 (Magma) [k: k in [3..6000] | ((EulerPhi(k) div 2) - 1) eq EulerPhi(k + 1)];
%Y A376338 Cf. A000010, A162857, A376337.
%K A376338 nonn
%O A376338 1,1
%A A376338 _Juri-Stepan Gerasimov_, Sep 20 2024