A182672 a(n) = the largest n-digit number with exactly 6 divisors, a(n) = 0 if no such number exists.

0, 99, 981, 9981, 99997, 999981, 9999988, 99999961, 999999981, 9999999908, 99999999964, 999999999927, 9999999999884, 99999999999932, 999999999999908, 9999999999999925, 99999999999999963, 999999999999999929, 9999999999999999999, 99999999999999999916

Offset

1,2

Comments

a(n) = the largest n-digit number of the form p^5 or p^2*q^1, (p, q = distinct primes), a(n) = 0 if no such number exists.

Links

Table of n, a(n) for n=1..20.

Formula

A000005(a(n)) = 6.

Mathematica

Table[k=10^n-1; While[k>10^(n-1) && DivisorSigma[0, k] != 6, k--]; If[k==10^(n-1), k=0]; k, {n, 20}]

Crossrefs

Cf. A182671 (smallest n-digit number with exactly 6 divisors).

Sequence in context: A061366 A177686 A135219 * A124113 A223157 A223155

Adjacent sequences: A182669 A182670 A182671 * A182673 A182674 A182675

Keyword

nonn,base

Author

Jaroslav Krizek, Nov 27 2010

Extensions

Extended by T. D. Noe, Nov 29 2010

Status

approved