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Smallest k such that each of the five factorials (5k)!, (5k+1)!, (5k+2)!, (5k+3)! and (5k+4)! has exactly 10^n trailing 0's. Zero, if no such k exists.
1

%I #12 Oct 06 2014 04:14:13

%S 1,9,81,801,8002,80001,800001,8000002,80000003,800000003,8000000003,

%T 80000000003,800000000002,8000000000003,80000000000004,0,0,0,

%U 800000000000000004,8000000000000000003,0,800000000000000000005,8000000000000000000006,0

%N Smallest k such that each of the five factorials (5k)!, (5k+1)!, (5k+2)!, (5k+3)! and (5k+4)! has exactly 10^n trailing 0's. Zero, if no such k exists.

%C a(n) = A173558(n)/5 or 0.

%H Hiroaki Yamanouchi, <a href="/A181581/b181581.txt">Table of n, a(n) for n = 0..100</a>

%H <a href="/index/Fa#factorial">Index to sequences related to factorial numbers</a>.

%Y Cf. A027868, A173558.

%K nonn

%O 0,2

%A _Lekraj Beedassy_, Nov 02 2010

%E a(15)-a(17) corrected and a(18)-a(23) added by _Hiroaki Yamanouchi_, Oct 06 2014