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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. 3

%I

%S 0,2,2,6,4,-1,6,686,8,666,2,22,444,2002,686,-1,464,646,666,646,2,6006,

%T 22,828,888,-1,2002,8886888,868,464,6,868,4224,66,646,-1,828,222,646,

%U 6006,4,22222,6006,68886,44,-1,828,282,4224,686,-1,42024,4004,424,8886888,-1,8008,68286,464,68086,6

%N 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 a(n) = -1 if and only if m ends with the digit 5.

%H P. De Geest, <a href="http://www.worldofnumbers.com/em36.htm">Smallest multipliers to make a number palindromic</a>.

%e For n = 30 we have m = 3; 3*2 = 6 is a palindrome with even digits, so a(30) = 6.

%o (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 Cf. A050782, A062293, A061816, A061906. Values of k are given in A061916.

%K base,sign,easy

%O 0,2

%A _Klaus Brockhaus_, Jun 25 2001

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Last modified July 12 06:48 EDT 2020. Contains 335657 sequences. (Running on oeis4.)