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a(n) = the largest n-digit number with exactly 6 divisors, a(n) = 0 if no such number exists.
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%I #8 Mar 30 2012 19:00:24

%S 0,99,981,9981,99997,999981,9999988,99999961,999999981,9999999908,

%T 99999999964,999999999927,9999999999884,99999999999932,

%U 999999999999908,9999999999999925,99999999999999963,999999999999999929,9999999999999999999,99999999999999999916

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

%C 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.

%F A000005(a(n)) = 6.

%t 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}]

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

%K nonn,base

%O 1,2

%A _Jaroslav Krizek_, Nov 27 2010

%E Extended by _T. D. Noe_, Nov 29 2010