OFFSET
1,1
COMMENTS
The n-th triangular number T(n) = n*(n+1)/2.
Triangular numbers of the form p*q + 2, where p and q are primes (not necessarily distinct).
LINKS
K. D. Bajpai and N. J. A. Sloane, Table of n, a(n) for n = 1..27939 [First 10000 terms from K. D. Bajpai]
EXAMPLE
a(2) = 28 = 7*(7+1)/2 = 28 - 2 = 26 = 2 * 13 is semiprime.
a(3) = 36 = 8*(8+1)/2 = 36 - 2 = 34 = 2 * 17 is semiprime.
MAPLE
MATHEMATICA
Select[Table[n*(n + 1)/2, {n, 200}], PrimeOmega[# - 2] == 2 &]
Select[Accumulate[Range[200]], PrimeOmega[#-2]==2&] (* Harvey P. Dale, Feb 21 2023 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
K. D. Bajpai, May 11 2014
STATUS
approved