From: utzoo!decvax!pur-ee!uiucdcs!kolstad
Newsgroups: net.math
Title: Rep-unit numbers of length 6k-1 - from a notesfile
Article-I.D.: uiucdcs.124
Posted: Thu May 13 10:33:32 1982
Received: Fri May 14 05:27:00 1982
Reply-To: \s

#N:uiucdcs:10300001:000:373
uiucdcs!kolstad    May 13 10:29:00 1982

My colleague recently asked me the following question:

Are all decimal numbers of length 6k-1 (k>0) consisting of all 1's
composite?  (e.g., 11111 = 41*271; 11111111111 = 21649 * 513239).

I have deduced a general function for the numbers:

			6k-1
	              10    - 1
          f(k) =     -----------
			  9

but this has helped me not.  Anyone got any suggestions?