A100915 - OEIS

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A100915

Numbers n such that n plus n-th semiprime is semiprime.

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: A274135 A163656 A134678 * A144922 A298864 A310667

Adjacent sequences: A100912 A100913 A100914 * A100916 A100917 A100918

KEYWORD

easy,nonn

AUTHOR

Ray Chandler, Nov 26 2004

STATUS

approved