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Any digit "d" (except the first two) is the absolute difference between x and y -> x=sum of the 2 digits standing immediately to the left of "d", y=sum of the 2 digits standing immediately to the right of "d".
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%I #5 Mar 31 2012 13:46:51

%S 0,1,2,120,121,201,2120,12120,121201,212012,12012120,12120121,

%T 20121201,212012120,1212012120,12120121201,21201212012,1201212012120,

%U 1212012120121,2012120121201,21201212012120,121201212012120

%N Any digit "d" (except the first two) is the absolute difference between x and y -> x=sum of the 2 digits standing immediately to the left of "d", y=sum of the 2 digits standing immediately to the right of "d".

%C To build such sequences, repeat one of those three patterns and insert commas accordingly: i) 0aaaa -> 0aaaa0aaaa0aaaa... -> 0,1,11,101,1110,11110... j) 0abab -> 0abab0abab0abab... -> 0,4,9,490,494,904,9490... k) 0a00a -> 0a00a0a00a0a00a... -> 0,700,70700,7070070,...

%e The first "2" is the absolute difference (0+1)-(1+2);

%e the following "1" is the result of (2+1)-(2+0);

%e the following "2" is the result of (1+2)-(0+1);

%e the following "0" is the result of (1+2)-(1+2)...

%K base,easy,nonn

%O 0,3

%A _Eric Angelini_, Feb 21 2005