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A115708
Semiprimes (A001358) whose digit reversal is a pentagonal number (A000326).
2
10, 15, 21, 22, 235, 287, 517, 529, 671, 1513, 1555, 2611, 5137, 5331, 6241, 7017, 7171, 7421, 7597, 7814, 10078, 10913, 15011, 15094, 15214, 15251, 20395, 20603, 21542, 23129, 24501, 24873, 25157, 26851, 27161, 51998, 53297, 54989, 55551
OFFSET
1,1
LINKS
EXAMPLE
235=5*47 is semiprime and 532 is the 19th pentagonal number.
MATHEMATICA
Select[Range[60000], PrimeOmega[#]==2&&IntegerQ[(1+Sqrt[1+24* IntegerReverse[#]])/ 6]&] (* Harvey P. Dale, Apr 27 2022 *)
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Giovanni Resta, Jan 31 2006
STATUS
approved

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Last modified September 19 20:04 EDT 2024. Contains 376014 sequences. (Running on oeis4.)