cp's OEIS Frontend

A087502 Smallest positive integer which when written in base n is doubled when the last digit is put first.

%I A087502 #17 Jan 07 2019 03:24:31
%S A087502 32,18,8,10993850,2129428800,21,5064320,105263157894736842,40,
%T A087502 64609423538,5712,65,58774271029236501660840264682112,67650,96,833,
%U A087502 586081355679130611935159482937228562988190880,133
%N A087502 Smallest positive integer which when written in base n is doubled when the last digit is put first.
%C A087502 a(n) is the smallest integer of the form x*(n^d-1)/(2n-1) for integer x and d, where 1 < x < n and d > 1. x is the last digit and d is the number of digits of a(n) in base n. - _Pontus von Brömssen_, Jan 06 2019
%H A087502 Pontus von Brömssen, <a href="/A087502/b087502.txt">Table of n, a(n) for n = 3..221</a>
%e A087502 a(10) = 105263157894736842 because 2*105263157894736842 = 210526315789473684 and no smaller number has this property. (Leading zeros are not allowed, otherwise 2*052631578947368421 = 105263157894736842 would be a smaller solution.)
%p A087502 A087502 := proc(n) local d,a; d := 1; a := n; while a>=n do d := d+1; a := denom((2^d-1)/(2*n-1)); od; return(max(2,a)*(n^d-1)/(2*n-1)); end proc;
%Y A087502 See A158877 for these numbers written in base n. Cf. A023094, A034089, A081463, A087502.
%K A087502 nonn,base
%O A087502 3,1
%A A087502 _Pontus von Brömssen_, Sep 10 2003