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
KEYWORD
nonn
AUTHOR
Zhi-Wei Sun, Nov 05 2012
STATUS
approved