%I #6 Dec 15 2017 17:34:57

%S 1,5,25,125,625,3125,15625,78125,-1,1953125,-1,-1,-1,-1,-1,-1,-1,-1,

%T 3814697265625,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,

%U 116415321826934814453125,-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) = largest such y, 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) = 25.

%Y Cf. A060376 (for values of x).

%K sign,base

%O 0,2

%A _Jason Earls_, Apr 02 2001