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A154541
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Numbers k such that reverse(k) is the number of divisors of k.
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0
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1, 2, 420, 23000, 441000, 89000000, 2340000000, 8210000000, 6160000000000, 25410000000000, 27600000000000, 42600000000000, 2930000000000000, 8440000000000000, 445000000000000000, 65110000000000000000, 227000000000000000000, 250200000000000000000, 449100000000000000000, 4932000000000000000000
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OFFSET
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1,2
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COMMENTS
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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.
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LINKS
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Table of n, a(n) for n=1..20.
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EXAMPLE
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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.
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MAPLE
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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
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PROG
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(PARI) isok(k) = fromdigits(Vecrev(digits(k))) == numdiv(k); \\ Michel Marcus, Jan 06 2019
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CROSSREFS
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Cf. A000005 (number of divisors), A004086 (reverse).
Sequence in context: A259562 A177321 A080392 * A119120 A332142 A109931
Adjacent sequences: A154538 A154539 A154540 * A154542 A154543 A154544
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KEYWORD
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nonn,base
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AUTHOR
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Avik Roy (avik_3.1416(AT)yahoo.co.in), Jan 11 2009
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EXTENSIONS
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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
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STATUS
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approved
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