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%I #28 Apr 14 2020 03:28:21
%S 2,7,17,71,6607,15313,91801,141689,443777,858463,1353593,2345479,
%T 726919199,2458927937,7425764663,37193744801,329683117297,
%U 973676004031,1294734832753,3825780992497,10360880429177
%N Prime(n) such that 2*n^2 - prime(n) is square.
%C The associated indices n are 1, 4, 7, 20, 854, 1789, 8869, 13157, 37247, 68234, ....
%C a(13) > 6*10^6. - _W. Edwin Clark_, Jul 25 2013
%C a(14) > 1.7 * 10^9. - _Robert Israel_, Apr 13 2020
%e 17 is in the sequence because 17 = prime(7) and 2*7^2 - 17 = 81 is square.
%p p:=1: count:= 0: R:= NULL: S:= NULL:
%p for i from 1 while count < 13 do
%p p:= nextprime(p);
%p if issqr(2*i^2-p) then
%p count:= count+1;
%p R:= R, p;
%p S:= S, i;
%p fi
%p od:
%p R; # _Robert Israel_, Apr 13 2020
%o (PARI) isok(p) = isprime(p) && issquare(2*primepi(p)^2 - p); \\ _Michel Marcus_, Apr 14 2020
%K nonn
%O 1,1
%A _Juri-Stepan Gerasimov_, Jul 29 2013
%E a(5)-a(11) from _Harvey P. Dale_, Jul 22 2013
%E a(12) from _W. Edwin Clark_, Jul 25 2013
%E a(13) from _Robert Israel_, Apr 13 2020
%E a(9) corrected and a(14)-a(21) added by _Giovanni Resta_, Apr 13 2020
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