OFFSET
1,1
COMMENTS
For prime indices p such that p(p+1)/2 is 'odd' see A002313 (primes of form 4n+1).
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..5000
Eric Weisstein's World of Mathematics, Triangular Number
FORMULA
MATHEMATICA
Select[Table[Prime[n] (Prime[n] + 1)/2, {n, 500}], OddQ[#] &] (* G. C. Greubel, Nov 03 2017 *)
With[{nn=400}, Select[Thread[{Accumulate[Range[nn]], Range[nn]}], OddQ[ #[[1]]] && PrimeQ[#[[2]]]&]][[All, 1]] (* Harvey P. Dale, Mar 06 2019 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Patrick De Geest, Oct 15 1998
STATUS
approved