OFFSET
0,1
COMMENTS
a(n) = 7 if 3^n has d digits where 3^d + 5^n == 0 (mod 7).
a(n) is the concatenation of 2^n and 3^n if n is in A268111.
EXAMPLE
a(5) = 19 because the concatenation of 2^5 and 3^5 is 32243 = 19 * 1697.
MAPLE
f:= proc(n) local b, v, F;
b:= 3^n;
v:= 2^n*10^(1+ilog10(b)) + b;
F:= select(type, ifactors(v, easy)[2][.., 1], integer);
if F <> [] then return min(F) fi;
min(ifactors(v)[2][..., 1]);
end proc;
map(f, [$0..90]);
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Robert Israel, Jul 12 2023
EXTENSIONS
Duplicated terms (former a(11)-a(20)) removed by Georg Fischer, May 23 2024
STATUS
approved