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A184481
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Semiprime centered triangular numbers.
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2
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4, 10, 46, 85, 166, 235, 274, 361, 514, 694, 901, 1135, 1219, 1306, 1585, 1891, 2461, 2839, 3106, 3385, 3826, 3979, 4135, 5311, 5674, 6049, 6241, 6835, 7246, 8551, 9481, 10966, 11485, 11749, 12286, 12559, 13969, 15151, 15454, 17335, 18649, 18985, 19666, 21421, 21781, 22879, 23626, 24385, 26734, 27949, 28774, 30034, 32194, 33079, 33526
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OFFSET
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1,1
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COMMENTS
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Numbers of the form 3*n*(n-1)/2 + 1 = p*q where p and q are primes, not necessarily distinct. This is to semiprimes A001358 as A125602 is to primes A000040.
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LINKS
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Table of n, a(n) for n=1..55.
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FORMULA
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A001358 INTERSECTION A005448.
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EXAMPLE
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a(3) = 3*6(6-1)/2 + 1 = 10 = 2 * 5.
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MATHEMATICA
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SemiprimeQ[n_] := Total[FactorInteger[n]][[2]] == 2; Select[Table[3*n (n - 1)/2 + 1, {n, 150}], SemiprimeQ]
Select[Table[(3n(n-1))/2+1, {n, 200}], PrimeOmega[#]==2&] (* Harvey P. Dale, May 13 2012 *)
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CROSSREFS
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Cf. A001358, A005448, A125602.
Sequence in context: A155223 A155219 A153985 * A149227 A149228 A149229
Adjacent sequences: A184478 A184479 A184480 * A184482 A184483 A184484
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KEYWORD
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nonn,easy
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AUTHOR
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Jonathan Vos Post, Feb 12 2011
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STATUS
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approved
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