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A034954
Odd triangular numbers with prime indices.
3
3, 15, 91, 153, 435, 703, 861, 1431, 1891, 2701, 4005, 4753, 5151, 5995, 6441, 9453, 11175, 12403, 15051, 16471, 18721, 19503, 26335, 27261, 29161, 33153, 36315, 38503, 39621, 43071, 49141, 50403, 56953, 61075, 62481, 69751, 75855, 79003
OFFSET
1,1
COMMENTS
For prime indices p such that p(p+1)/2 is 'odd' see A002313 (primes of form 4n+1).
LINKS
Eric Weisstein's World of Mathematics, Triangular Number
FORMULA
a(n) = A000217(A002313(n)). - Michel Marcus, Nov 04 2017
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