OFFSET
1,2
COMMENTS
a(n) = -1 if n == 3 (mod 5).
LINKS
Robert Israel, Table of n, a(n) for n = 1..5000
FORMULA
If a(n) > 0, A000422(a(n)) == 0 (mod n).
EXAMPLE
a(9) = 8 because 87654321 is divisible by 9, but none of 1, 21, 321, ..., 7654321 are divisible by 9.
MAPLE
F:= proc(n) local k, x, l;
if n mod 5 = 0 then return -1 fi;
x:= 1; l:= 1;
for k from 2 do
x:= k * 10^l+x mod n;
if x = 0 then return k fi;
l:= l + 1 + ilog10(k);
od;
end proc;
F(1):= 1:
map(F, [seq(i, i=1..201, 2)]);
CROSSREFS
KEYWORD
sign,base
AUTHOR
Robert Israel, Oct 24 2025
STATUS
approved