A225562 a(n) = smallest k such that n is the n-th largest divisor of k.

1, 4, 15, 20, 30, 48, 84, 160, 144, 210, 462, 240, 624, 1134, 480, 864, 1836, 720, 8740, 840, 1512, 2376, 4968, 2400, 3900, 3120, 4536, 4032, 15312, 2520, 17856, 5280, 6930, 10710, 15400, 7200, 47952, 17100, 12480, 7920, 72324, 9240, 43344, 16632, 20790

Offset: 1

Author

Irina Gerasimova, May 13 2013

Keywords

• nonn

Comments

The smallest row k such that n is the n-th entry in the triangle A056538 of divisors in reverse order.

Is a(n) defined for every n ? - Giovanni Resta, May 15 2013

Examples

			a(6) = 48 because the divisors of 48 are {48, 24, 16, 12, 8, 6, 4, 3, 2, 1} and 6 is the 6th divisor of 48.
		

Links

• Alois P. Heinz and Zak Seidov, Table of n, a(n) for n = 1..1000 (first 200 terms from Alois P. Heinz)

Programs

Maple

with(numtheory):
a:= proc(n) local k;
      for k from n by n while tau(k)`)[n]<>n do od; k
    end:
seq(a(n), n=1..50);  # Alois P. Heinz, May 29 2013

Mathematica

a[n_] := Block[{k = 1, d}, While[Length[d = Reverse@ Divisors@ k] < n || n != d[[n]], k++]; k]; Array[a, 20] (* Giovanni Resta, May 15 2013 *)

Extensions

a(13)-a(45) from Giovanni Resta, May 15 2013