cp's OEIS Frontend

A061915 Obtain m by omitting trailing zeros from n; a(n) = smallest multiple k*m which is a palindrome with even digits, or -1 if no such multiple exists.

%I A061915 #7 Feb 13 2025 21:15:25
%S A061915 0,2,2,6,4,-1,6,686,8,666,2,22,444,2002,686,-1,464,646,666,646,2,6006,
%T A061915 22,828,888,-1,2002,8886888,868,464,6,868,4224,66,646,-1,828,222,646,
%U A061915 6006,4,22222,6006,68886,44,-1,828,282,4224,686,-1,42024,4004,424,8886888,-1,8008,68286,464,68086,6
%N A061915 Obtain m by omitting trailing zeros from n; a(n) = smallest multiple k*m which is a palindrome with even digits, or -1 if no such multiple exists.
%C A061915 a(n) = -1 if and only if m ends with the digit 5.
%H A061915 P. De Geest, <a href="https://www.worldofnumbers.com/em36.htm">Smallest multipliers to make a number palindromic</a>.
%e A061915 For n = 30 we have m = 3; 3*2 = 6 is a palindrome with even digits, so a(30) = 6.
%o A061915 (ARIBAS) stop := 500000; for n := 0 to 60 do k := 1; test := true; while test and k < stop do mp := omit_trailzeros(n)*k; if test := not all_even(mp) or mp <> int_reverse(mp) then inc(k); end; end; if k < stop then write(mp," "); else write(-1," "); end; end;
%Y A061915 Cf. A050782, A062293, A061816, A061906. Values of k are given in A061916.
%K A061915 base,sign,easy
%O A061915 0,2
%A A061915 _Klaus Brockhaus_, Jun 25 2001