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A100915 Numbers n such that n plus n-th semiprime is semiprime. 5
4, 6, 9, 12, 16, 18, 19, 20, 24, 29, 31, 34, 35, 39, 40, 44, 46, 49, 51, 54, 55, 72, 73, 76, 79, 80, 81, 84, 87, 91, 93, 94, 96, 98, 110, 113, 116, 120, 128, 130, 136, 137, 148, 150, 154, 159, 165, 168, 170, 172, 175, 188, 190, 191, 199, 200, 206, 215, 217, 220, 230 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This is the semiprime analog of A064402.

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Eric Weisstein, World of Mathematics, Semiprime.

FORMULA

a(n) = A100466(n) - A100916(n) = A100466(n) - A001358(A100915(n)).

EXAMPLE

a(3) = 9 because 9 + semiprime(9) = 9 + 25 = 34 is semiprime.

MATHEMATICA

With[{c=Select[Range[1000], PrimeOmega[#]==2&]}, Transpose[Select[Thread[ {c, Range[ Length[c]]}], PrimeOmega[Total[#]]==2&]][[2]]] (* Harvey P. Dale, Oct 25 2011 *)

CROSSREFS

Cf. A001358, A064402, A100493, A100466, A100467, A100916.

Sequence in context: A024675 A274135 A163656 * A144922 A298864 A310667

Adjacent sequences:  A100912 A100913 A100914 * A100916 A100917 A100918

KEYWORD

easy,nonn

AUTHOR

Ray Chandler, Nov 26 2004

STATUS

approved

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Last modified December 6 12:36 EST 2021. Contains 349563 sequences. (Running on oeis4.)