OFFSET
1,1
COMMENTS
The n-th triangular number T(n) = n*(n+1)/2.
Triangular numbers of the form p*q - 2 and r*s + 2 where p, q, r and s are primes.
LINKS
K. D. Bajpai, Table of n, a(n) for n = 1..6565
EXAMPLE
a(1) = 36 = 8*(8+1)/2 = 36 + 2 = 38 = 2 * 19 and 36 - 2 = 34 = 2 * 17 both are semiprimes.
a(2) = 120 = 15*(15+1)/2 = 120 + 2 = 122 = 2 * 61 and 120 - 2 = 118 = 2 * 59 both are semiprimes.
MAPLE
MATHEMATICA
Select[Table[n*(n + 1)/2, {n, 500}], PrimeOmega[# + 2] == 2 && PrimeOmega[# - 2] == 2 &]
Select[Accumulate[Range[300]], PrimeOmega[#+{2, -2}]=={2, 2}&] (* Harvey P. Dale, Apr 21 2016 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
K. D. Bajpai, May 11 2014
STATUS
approved