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A115709
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Pentagonal numbers (A000326) whose digit reversal is a semiprime (A001358).
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5
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12, 22, 51, 176, 330, 532, 590, 715, 782, 925, 1162, 1247, 1335, 1426, 1717, 3151, 3290, 4187, 5551, 7107, 7315, 7957, 10542, 10795, 11051, 11837, 12376, 14950, 15251, 15555, 15862, 16172, 17120, 19097, 19780, 20126, 22265, 24130, 24512, 26467, 26867, 30175
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OFFSET
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1,1
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LINKS
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EXAMPLE
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532 is the 19th pentagonal number and 235=5*47 is semiprime.
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MATHEMATICA
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Select[PolygonalNumber[5, Range[200]], PrimeOmega[IntegerReverse[#]]==2&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jan 27 2019 *)
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PROG
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(Python)
from sympy import factorint
def ok(p): return sum(e for e in factorint(int(str(p)[::-1])).values()) == 2
print([p for p in (n*(3*n-1)//2 for n in range(143)) if ok(p)]) # Michael S. Branicky, Dec 22 2021
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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