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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
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified December 5 20:39 EST 2022. Contains 358589 sequences. (Running on oeis4.)