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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
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.
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: A177321 A365520 A080392 * A119120 A373552 A332142
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