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 A154541 Numbers k such that reverse(k) is the number of divisors of k. 0
 1, 2, 420, 23000, 441000, 89000000, 2340000000, 8210000000, 6160000000000, 25410000000000, 27600000000000, 42600000000000, 2930000000000000, 8440000000000000, 445000000000000000, 65110000000000000000, 227000000000000000000, 250200000000000000000, 449100000000000000000, 4932000000000000000000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The larger terms of the sequence are believed to end in zeros. It is assumed that the number of divisors of any number is usually significantly smaller than the number. LINKS EXAMPLE 420 is a term because reverse(420) = 24 and 420 has 24 factors: 1, 2, 3, 4, 5, 6, 7, 10, 12, 14, 15, 20, 21, 28, 30, 35, 42, 60, 70, 84, 105, 140, 210, 420. MAPLE with(numtheory): rev := proc (n) local nn: nn := convert(n, base, 10): add(nn[j]*10^(nops(nn)-j), j = 1 .. nops(nn)) end proc: a := proc (n) if rev(n) = tau(n) then n else end if end proc: seq(a(n), n = 1 .. 25000); # Emeric Deutsch, Jan 15 2009 PROG (PARI) isok(k) = fromdigits(Vecrev(digits(k))) == numdiv(k); \\ Michel Marcus, Jan 06 2019 CROSSREFS Cf. A000005 (number of divisors), A004086 (reverse). Sequence in context: A259562 A177321 A080392 * A119120 A332142 A109931 Adjacent sequences: A154538 A154539 A154540 * A154542 A154543 A154544 KEYWORD nonn,base AUTHOR Avik Roy (avik_3.1416(AT)yahoo.co.in), Jan 11 2009 EXTENSIONS a(5) from Emeric Deutsch, Jan 15 2009 a(6)-a(15) from Donovan Johnson, Jun 14 2009 Terms a(16) onward from Max Alekseyev, Feb 16 2011 Edited by Jon E. Schoenfield, Jan 06 2019 STATUS approved

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Last modified March 26 15:32 EDT 2023. Contains 361549 sequences. (Running on oeis4.)