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 A225774 #18 Nov 02 2024 06:49:38 %S A225774 13,43,109,151,883,2143,34360131583 %N A225774 Primes p such that sigma(sigma(p)) = 2*phi(p); sigma(n) = A000203(n); phi(n) = A000010(n). %C A225774 There are no other terms <= 2*10^8. %C A225774 Also primes p such that sigma(p + 1) = 2*phi(p) = 2p - 2. %C A225774 Also primes p such that sigma(sigma(p)) - sigma(p) - p = -3. The only composite number <= 2*10^8 with this property is the number 4. %C A225774 Subsequence of primes in A135241 (numbers k such that sigma(sigma(k)) = 2*phi(k)). %C A225774 Primes p such that sigma(p+1)-2*(p+1) = -4. - _Donovan Johnson_, Aug 01 2013 %e A225774 sigma(sigma(13)) = 2*phi(13) = 24. %Y A225774 Cf. A000203, A000010, A135241, A125246. %K A225774 nonn,more %O A225774 1,1 %A A225774 _Jaroslav Krizek_, Jul 26 2013 %E A225774 a(7) from _Donovan Johnson_, Aug 01 2013