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A263720
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Palindromic numbers such that the sum of the digits equals the number of divisors.
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1
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1, 2, 11, 22, 101, 202, 444, 525, 828, 1111, 2222, 4884, 5445, 5775, 12321, 13431, 18081, 21612, 24642, 26862, 31213, 44244, 44844, 51415, 52425, 56265, 62426, 80008, 86868, 89298, 99099, 135531, 162261, 198891, 217712, 237732, 301103, 343343, 480084, 486684, 512215, 521125
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OFFSET
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1,2
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COMMENTS
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The only known palindromic primes whose sum of digits equals the numbers of divisors (primes of the form 10^k + 1) are 2,11,101.
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LINKS
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Eric Weisstein's World of Mathematics, Digit Sum
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EXAMPLE
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a(3) = 11, 11 is the palindromic number, digitsum(11) = 1 + 1 = 2, sigma_0(11) = 2.
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MATHEMATICA
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fQ[n_] := Block[{d = IntegerDigits@ n}, And[d == Reverse@ d, Total@ d == DivisorSigma[0, n]]]; Select[Range[2^19], fQ] (* Michael De Vlieger, Oct 27 2015 *)
Select[Range[600000], PalindromeQ[#]&&Total[IntegerDigits[#]] == DivisorSigma[ 0, #]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Aug 28 2019 *)
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PROG
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(PARI) lista(nn) = {for(n=1, nn, my(d = digits(n)); if ((Vecrev(d) == d) && (numdiv(n) == sumdigits(n)), print1(n, ", ")); ); } \\ Michel Marcus, Oct 25 2015
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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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