%I
%S 13,19,43,53,89,103,151,229,251,349,433,463,593,701,739,859,1033,1223,
%T 1429,1483,1709,1889,1951,2143,2699,3001,3079,3319,3739,4003,4093,
%U 4463,4751,5563,5669,6553,7019,7873,8513,9043,10009,10151,10729,11173,11779
%N Primes of the form k(k+1)/22 (i.e., two less than triangular numbers).
%C Equal to primes of the form (k^217)/8. Also equal to primes p such that 8*p+17 is a square.  _Chai Wah Wu_, Jul 14 2014
%H Chai Wah Wu, <a href="/A124199/b124199.txt">Table of n, a(n) for n = 1..2000</a>.
%e The (first five triangular numbers)2 are: 1,1,4,8,13. So a(1)=13 is the first prime of this form.
%t Pick[ #1, PrimeQ[ #1]]&[((1/2)*#1*(#1 + 1)  2 & ) /@ Range[180]]
%o (Python)
%o import sympy
%o [n*(n+1)/22 for n in range(10**6) if isprime(n*(n+1)/22)] # _Chai Wah Wu_, Jul 14 2014
%Y Cf. A055472.
%K easy,nonn
%O 1,1
%A Peter Pein (petsie(AT)dordos.net), Dec 07 2006
