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A184481 Semiprime centered triangular numbers. 2
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

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.

LINKS

Table of n, a(n) for n=1..55.

FORMULA

A001358 INTERSECTION A005448.

EXAMPLE

a(3) = 3*6(6-1)/2 + 1 = 10 = 2 * 5.

MATHEMATICA

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 *)

CROSSREFS

Cf. A001358, A005448, A125602.

Sequence in context: A155223 A155219 A153985 * A149227 A149228 A149229

Adjacent sequences:  A184478 A184479 A184480 * A184482 A184483 A184484

KEYWORD

nonn,easy

AUTHOR

Jonathan Vos Post, Feb 12 2011

STATUS

approved

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Last modified May 27 11:51 EDT 2017. Contains 287205 sequences.