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a(n) = the largest n-digit number with exactly 10 divisors, a(n) = 0 if no such number exists.
1

%I #8 Mar 30 2012 19:00:24

%S 0,80,976,9904,99952,999952,9999952,99999824,999999536,9999999824,

%T 99999999536,999999999567,9999999999963,99999999999728,

%U 999999999999856,9999999999998896,99999999999999824,999999999999999952,9999999999999999856,99999999999999999568

%N a(n) = the largest n-digit number with exactly 10 divisors, a(n) = 0 if no such number exists.

%C a(n) = the largest n-digit number of the form p^9 or p^4*q (p, q distinct primes), a(n) = 0 if no such number exists.

%F A000005(a(n)) = 10.

%F a(n) >= A182679(n).

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

%Y Cf. A030628, A182679.

%K nonn,base

%O 1,2

%A _Jaroslav Krizek_, Nov 27 2010

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