%I #18 Sep 25 2014 21:46:58

%S 2,3,7,19,29,31,37,47,53,59,83,103,109,113,131,137,139,149,167,193,

%T 197,199,223,227,233,251,271,277,281,283,307,311,317,337,367,373,383,

%U 389,401,419,421,439,449,457,467

%N Primes p such that 7 is a square mod p.

%C Also, only entries p=1 (mod 4) of the sequence are squares mod 7 (from the quadratic reciprocity law). - _Lekraj Beedassy_, Jul 21 2004

%C 2 and 7 together with primes congruent to +/- {1, 3, 9} mod 28. - _Eric M. Schmidt_, Jan 24 2014

%C Least numbers m such that 7+m*a(n) is a square: 1, 3, 6, 3, 1, 3, 2, 6, 9, 6, 3, 6, 2, 9, 3, 1, ... - _Zak Seidov_, Sep 25 2014

%H Vincenzo Librandi, <a href="/A038878/b038878.txt">Table of n, a(n) for n = 1..1000</a>

%t Select[Prime[Range[100]], JacobiSymbol[7, #] != -1 &] (* _Vincenzo Librandi_, Sep 07 2012 *)

%t Union[{2,7},Select[Flatten[Table[{1,3,9,19,25,27}+28*k,{k,0,10}]],PrimeQ]](* _Zak Seidov_, Sep 25 2014 *)

%K nonn

%O 1,1

%A _N. J. A. Sloane_.