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 A242344 Triangular numbers T such that T-2 is semiprime. 1
 6, 28, 36, 120, 136, 171, 276, 300, 325, 528, 561, 780, 820, 903, 1081, 1128, 1176, 1275, 1540, 1596, 1653, 2080, 2211, 2415, 2485, 2556, 2775, 3160, 3240, 3403, 3655, 3828, 4371, 4851, 5151, 5253, 5356, 5460, 5995, 6105, 6328, 6441, 6903, 7381, 7503, 8001, 8256 (list; graph; refs; listen; history; text; internal format)
 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 with(numtheory): A242344:= proc()local t; t:=x*(x+1)/2; if bigomega(t-2)=2 then  RETURN (t); fi; end: seq(A242344(), x=1..200); MATHEMATICA Select[Table[n*(n + 1)/2, {n, 200}], PrimeOmega[# - 2] == 2 &] CROSSREFS Cf. A001358, A000217, A063637, A063638. Sequence in context: A272971 A117948 A252234 * A247111 A071834 A295078 Adjacent sequences:  A242341 A242342 A242343 * A242345 A242346 A242347 KEYWORD nonn AUTHOR K. D. Bajpai, May 11 2014 STATUS approved

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Last modified October 18 03:22 EDT 2019. Contains 328135 sequences. (Running on oeis4.)