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%I #6 Dec 15 2017 17:34:57
%S 1,2,4,8,16,32,64,128,-1,512,-1,-1,-1,-1,-1,-1,-1,-1,262144,-1,-1,-1,
%T -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,8589934592,-1,-1,-1,-1,-1,-1,-1,-1,
%U -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1
%N If 10^n can be written as x*y where the digits of x and y are all nonzero, then let a(n) = smallest such x, otherwise a(n) = -1.
%C According to Ogilvy and Anderson, 10^33 is the highest known power of ten that can be expressed as the product of two zero-free factors. "If there is another one, it is greater than 10^5000." p. 89
%D C. Stanley Ogilvy and John T. Anderson, Excursions in Number Theory, Oxford University Press, 1966, p. 89.
%D Rudolph Ondrejka, Nonzero factors of 10^n, Recreational Mathematics Magazine, no. 6 (1961), p. 59.
%e 10^2 = 4 * 25, so a(2) = 4.
%Y Cf. A060391 (for values of y).
%K sign,base
%O 0,2
%A _Jason Earls_, Apr 02 2001
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