A191915 - OEIS

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A191915

Semiprimes p*q with p < q such that the concatenation p||q is again a product of two distinct primes.

15, 26, 34, 35, 38, 55, 57, 69, 74, 85, 87, 91, 94, 95, 106, 118, 119, 123, 134, 161, 185, 202, 206, 209, 213, 215, 217, 221, 254, 259, 265, 295, 298, 303, 309, 314, 321, 334, 339, 346, 362, 365, 371, 377, 381, 382, 393, 395, 398, 407, 415, 417, 437, 445

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

COMMENTS

A subsequence of A191913 and of A191911.

LINKS

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

MATHEMATICA

okQ[n_]:=Module[{fd=FromDigits[Flatten[IntegerDigits/@ Transpose[ FactorInteger[ n]][[1]]]]}, PrimeOmega[fd]==PrimeNu[fd]==2]; With[ {sps = Select[Range[500], PrimeOmega[#] == PrimeNu[#] == 2 &]}, Select[ sps, okQ]] (* Harvey P. Dale, Nov 22 2011 *)

PROG

(PARI) for(i=1, 500, is_A006881(i)|next; f=factor(i); is_A006881(eval(Str(f[1, 1], f[2, 1]))) & print1(i", "))

CROSSREFS

Sequence in context: A342221 A074974 A191913 * A359547 A189045 A366961

Adjacent sequences: A191912 A191913 A191914 * A191916 A191917 A191918

KEYWORD

nonn,base

AUTHOR

M. F. Hasler, Jun 19 2011

STATUS

approved