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A100916
Sum of a semiprime and its semiprime index is a new semiprime.
4
10, 15, 25, 34, 46, 51, 55, 57, 69, 86, 91, 95, 106, 119, 121, 133, 141, 145, 155, 161, 166, 217, 218, 226, 247, 249, 253, 262, 274, 291, 298, 299, 302, 305, 341, 358, 365, 382, 407, 413, 445, 446, 481, 485, 501, 515, 533, 538, 543, 551, 559, 614, 623, 626
OFFSET
1,1
COMMENTS
This is the semiprime analog of A061067.
LINKS
Eric Weisstein, World of Mathematics, Semiprime.
FORMULA
a(n) = A100466(n) - A100915(n) = A001358(A100915(n)).
EXAMPLE
a(1) = 10 because 10 = semiprime(4) and semiprime(4) + 4 = 14 is
semiprime.
a(2) = 15 because 15 = semiprime(6) and semiprime(6) + 6 = 21 is
semiprime.
MATHEMATICA
Module[{sp=Select[Range[1000], PrimeOmega[#]==2&], len}, len=Length[sp]; Select[ Thread[{sp, Range[len]}], PrimeOmega[Total[#]]==2&]][[All, 1]] (* Harvey P. Dale, Jan 13 2019 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Ray Chandler, Nov 26 2004
STATUS
approved