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 A071621 #8 Dec 04 2024 11:39:05
%S A071621 13,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97,101,103,107,
%T A071621 109,113,127,131,137,139,149,151,157,163,167,173,179,181,191,193,197,
%U A071621 199,211,223,227,229,233,239,241,251,257,263,269,271,277,281,283,293
%N A071621 Primes that can be written as "a * b + c * d", where a, b, c and d are also primes.
%C A071621 As size in the Mathematica coding is increased, the primes not previously covered will probably be forthcoming. Conjecture: Only the primes 2, 3, 5, 7, 11 and 17 are not representable by this form.
%H A071621 Charles R Greathouse IV, <a href="/A071621/b071621.txt">Table of n, a(n) for n = 1..10000</a>
%e A071621 13 = 2*2 + 3*3, so 13 belongs to the sequence.
%t A071621 size = 15; Select[ Union[ Flatten[ Table[ ppp = Prime[i]Prime[j] + Prime[k]Prime[l]; If[ PrimeQ[ppp], Print[{Prime[i], Prime[j], Prime[k], Prime[l], ppp}]]; ppp, {i, size}, {j, size}, {k, size}, {l, size} ]]], PrimeQ]
%t A071621 Take[Select[Union[Total[Times@@@TakeDrop[#,2]]&/@Tuples[Prime[Range[15]],4]],PrimeQ],60] (* _Harvey P. Dale_, Dec 04 2024 *)
%o A071621 (PARI) list(lim)=my(v=vectorsmall(lim\1),u=List(),t); forprime(p=3,lim\2-2, forprime(q=2,min(p,(lim-4)\p), t=p*q; forprime(r=2,(lim-t)\2, v[t+2*r]=1))); forprime(i=1,lim,if(v[i],listput(u,i))); v=0; Set(u) \\ _Charles R Greathouse IV_, Nov 05 2015
%o A071621 (PARI) a(n)=if(n>1,prime(n+6),13) \\ _Charles R Greathouse IV_, Nov 05 2015
%K A071621 nonn
%O A071621 1,1
%A A071621 Arnoud Buzing (arnoudb(AT)wolfram.com), Jun 21 2002
%E A071621 Edited by _Robert G. Wilson v_, Jun 25 2002