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 A121051 #10 Oct 28 2017 09:48:41 %S A121051 15,39,55,87,95,111,119,143,159,183,215,247,287,295,303,319,327,335, %T A121051 391,407,415,447,471,511,519,527,535,543,551,559,583,591,623,655,671, %U A121051 679,687,695,703,767,791,799,807,815,831,871,879,895,943,951,959,1007 %N A121051 Semiprimes which are sums of 4 but no fewer nonzero squares. %C A121051 Semiprime analog of A007522 Primes of form 8n+7. These semiprimes must all be numbers of the form 4^i(8j+7), i >= 0, j >= 0. However, for positive i, 4^i(8j+7) has more than 2 prime factors (with multiplicity). Hence from Legendre's corollary to Lagrange's Four-Square Theorem, this sequence is precisely Semiprimes of the form 8*k+7. %t A121051 Select[8*Range[200]+7,PrimeOmega[#]==2&] (* _Harvey P. Dale_, Oct 28 2017 *) %Y A121051 A001358 intersect A004215. %Y A121051 Cf. A001358, A004215, A007522. %K A121051 easy,nonn,less %O A121051 1,1 %A A121051 _Jonathan Vos Post_, Aug 08 2006 %E A121051 a(15)-a(52) from _Giovanni Resta_, Jun 13 2016