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 A232444 #24 Jul 23 2016 11:49:57 %S A232444 2,4,64,289,729,15625,7091569,7778521,11607649,15912121,43546801, %T A232444 56957209,138980521,143688169,171845881,210801361,211673401,253541929, %U A232444 256224049,275792449,308810329,329386201,357172201,408807961,499477801,531625249,769341169,1073741824,1260747049 %N A232444 Numbers n such that sigma(n) and sigma(n^2) are primes. %C A232444 Intersection of A023194 and A055638. %C A232444 Sigma(n) = A000203(n) = sum of divisors of n. %C A232444 Terms a(2)...a(29) are squares of 2, 8, 17, 27, 125, 2663, 2789, 3407, 3989, 6599, 7547, 11789, 11987, 13109, 14519, 14549, 15923, 16007, 16607, 17573, 18149, 18899, 20219, 22349, 23057, 27737, 32768, 35507. %H A232444 Donovan Johnson and Chai Wah Wu, <a href="/A232444/b232444.txt">Table of n, a(n) for n = 1..10385</a> [Terms from 1 to 500 from Donovan Johnson] %e A232444 4 is in the sequence because both sigma(4)=7 and sigma(4^2)=31 are primes. %o A232444 (PARI) isok(n) = isprime(sigma(n)) && isprime(sigma(n^2)); \\ _Michel Marcus_, Nov 26 2013 %o A232444 (Python) %o A232444 from sympy import isprime, divisor_sigma %o A232444 A232444_list = [2]+[n for n in (d**2 for d in range(1,10**4)) if isprime(divisor_sigma(n)) and isprime(divisor_sigma(n**2))] # _Chai Wah Wu_, Jul 23 2016 %Y A232444 Cf. A000203, A023194, A055638. %K A232444 nonn %O A232444 1,1 %A A232444 _Alex Ratushnyak_, Nov 24 2013 %E A232444 a(6)-a(12) from _Michel Marcus_, Nov 26 2013 %E A232444 a(13)-a(29) from _Alex Ratushnyak_, Nov 26 2013