OFFSET
1,1
COMMENTS
The n-th triangular number T(n) = n*(n+1)/2 = A000217(n).
Triangular numbers of the form p*q - 2, where p and q are primes.
The indices of these triangular numbers are 8, 10, 13, 15, 17, 23, 24, 25, 29, 34, 42, 45, 49, 57, 58, 61, 63, 65, 70, 71, 77, 80, 86, 90, 94, 95, 98, 102, 104, 105, 109, 110, 111, 113, 114, 117, 118, 119, 120, 126, 128, 133, 134, 135, 138, 145, ... - Wolfdieter Lang, May 13 2014
LINKS
K. D. Bajpai, Table of n, a(n) for n = 1..10000
EXAMPLE
a(1) = 36 = 8*(8+1)/2 = 36 + 2 = 38 = 2 * 19 is semiprime.
a(2) = 55 = 10*(10+1)/2 = 55 + 2 = 57 = 3 * 19 is semiprime.
MAPLE
MATHEMATICA
Select[Table[n/2*(n + 1), {n, 200}], PrimeOmega[# + 2] == 2 &]
CROSSREFS
KEYWORD
nonn
AUTHOR
K. D. Bajpai, May 11 2014
STATUS
approved