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 A218771 Primes of the form p^2 + 3pq + q^2 with p and q prime. 4
 31, 59, 79, 179, 191, 229, 251, 311, 389, 401, 479, 491, 541, 569, 719, 809, 971, 1019, 1061, 1109, 1151, 1249, 1301, 1409, 1451, 1499, 1619, 1931, 1949, 2111, 2141, 2339, 2591, 2609, 2711, 2801, 2939, 3089, 3371, 3389, 3449, 3881, 4021, 4091, 4211, 4391, 4451, 4679, 5039, 5051 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS It is easy to see that a(n) is congruent to 1 or 9 modulo 10. For each n there is a unique pair of primes p < q such that p^2 + 3pq + q^2 = a(n). This sequence is of particular interest due to Zhi-Wei Sun's surprising conjecture related to A218754. That conjecture implies that this sequence is infinite. LINKS Zhi-Wei Sun, Table of n, a(n) for n = 1..10000 Zhi-Wei Sun, Conjectures involving primes and quadratic forms, arXiv preprint arXiv:1211.1588, 2012. EXAMPLE a(1)=31 since 2^2 + 3*2*3 + 3^2 = 31 and 2,3,31 are prime. MATHEMATICA SQ[n_]:=IntegerQ[Sqrt[n]] i=0; Do[Do[If[SQ[4Prime[n]+5Prime[k]^2] && PrimeQ[(Sqrt[4Prime[n] + 5Prime[k]^2] - 3Prime[k])/2] == True, i=i+1; Print[i, " ", Prime[n]]; Goto[aa]], {k, 1, PrimePi[Sqrt[Prime[n]/5]]}]; Label[aa]; Continue, {n, 1, 1000000}] PROG (PARI) list(lim)=my(v=List(), t); forprime(p=2, sqrtint(lim\4), forprime(q=p+1, sqrt(lim-p^2), if(isprime(t=p^2+3*p*q+q^2), listput(v, t), if(t>lim, break)))); vecsort(Vec(v)) \\ Charles R Greathouse IV, Nov 05 2012 (PARI) is_A218771(n, v=0)={ my(r, c=0); isprime(n) & forprime( q=1, sqrtint(n\5), issquare(4*n+5*q^2, &r) || next; isprime((r-3*q)/2) || next; v || return(1); v>1 & print1([q, (r-3*q)/2]", "); c++); c}  \\ - M. F. Hasler, Nov 05 2012 CROSSREFS Cf. A038872, A218754, A218585, A218654, A218656. Sequence in context: A293345 A096453 A218794 * A109356 A061251 A166604 Adjacent sequences:  A218768 A218769 A218770 * A218772 A218773 A218774 KEYWORD nonn AUTHOR Zhi-Wei Sun, Nov 05 2012 STATUS approved

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Last modified April 24 18:03 EDT 2019. Contains 322430 sequences. (Running on oeis4.)