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A242343
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Triangular numbers T such that (T+2) is semiprime.
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2
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36, 55, 91, 120, 153, 276, 300, 325, 435, 595, 903, 1035, 1225, 1653, 1711, 1891, 2016, 2145, 2485, 2556, 3003, 3240, 3741, 4095, 4465, 4560, 4851, 5253, 5460, 5565, 5995, 6105, 6216, 6441, 6555, 6903, 7021, 7140, 7260, 8001, 8256, 8911, 9045, 9180, 9591, 10585
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OFFSET
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1,1
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COMMENTS
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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
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LINKS
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EXAMPLE
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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.
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MAPLE
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with(numtheory): A242343:= proc()local t; t:=x/2*(x+1); if bigomega(t+2)=2 then RETURN (t); fi; end: seq(A242343 (), x=1..200);
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MATHEMATICA
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Select[Table[n/2*(n + 1), {n, 200}], PrimeOmega[# + 2] == 2 &]
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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