A060391 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.

1, 5, 25, 125, 625, 3125, 15625, 78125, -1, 1953125, -1, -1, -1, -1, -1, -1, -1, -1, 3814697265625, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 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

Offset

0,2

Comments

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

References

C. Stanley Ogilvy and John T. Anderson, Excursions in Number Theory, Oxford University Press, 1966, p. 89.

Rudolph Ondrejka, Nonzero factors of 10^n, Recreational Mathematics Magazine, no. 6 (1961), p. 59.

Links

Table of n, a(n) for n=0..68.

Example

10^2 = 4 * 25, so a(2) = 25.

Crossrefs

Cf. A060376 (for values of x).

Sequence in context: A335506 A129066 A102169 * A000351 A050735 A195948

Adjacent sequences: A060388 A060389 A060390 * A060392 A060393 A060394

Keyword

sign,base

Author

Jason Earls, Apr 02 2001

Status

approved