Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #73 Jan 17 2022 10:03:10
%S 1,11,111,121,1111,1221,1331,11111,12221,12321,13431,14641,111111,
%T 122221,123321,134431,135531,147741,161051,1111111,1222221,1233321,
%U 1234321,1344431,1356531,1367631,1478741,1490841,1625151,1771561,11111111,12222221,12333321
%N Numbers which are products of repunits.
%C The number of terms below 10^n is A216053(n)-1 for 1 <= n <= 25, but not for larger n. - _Rémy Sigrist_, May 28 2019
%C The product of repunits is not necessarily palindromic, see A339676. - _Bernard Schott_, Apr 02 2021
%H Robert Israel, <a href="/A308365/b308365.txt">Table of n, a(n) for n = 1..10000</a>
%e a(11) = 13431 is in the sequence since it is the product of repunits (11^2*111).
%p d:= 10: # for terms < 10^d
%p N:= 10^d:
%p S:= {1}:
%p for m from 2 to d do
%p r:= (10^m-1)/9;
%p k:= floor(log[r](N));
%p V:= S;
%p for i from 1 to k do
%p V:= select(`<`,map(`*`,V,r),N);
%p S:= S union V
%p od;
%p od:
%p sort(convert(S,list)); # _Robert Israel_, Nov 26 2020
%Y Cf. A002275 (repunits), A083278 (repunit powers), A216053, A339676 (nonpalindromic terms).
%K nonn,base
%O 1,2
%A _Sergio Pimentel_, May 22 2019
%E Missing a(25) = 1356531 inserted by _Ilya Gutkovskiy_, Apr 14 2020