%N Pick any pair of "4" digits in the sequence. Those two "4"s are separated by k digits. This is the lexicographically earliest sequence of distinct terms in which all the resulting values of k are distinct.
%C The sequence starts with a(1)=0. It is then always extended with the smallest integer not yet present and not leading to a contradiction (which would mean producing a value of k already seen).
%H Eric Angelini, <a href="/A273882/b273882.txt">Table of n, a(n) for n = 1..1011</a>
%Y See A273376 for the equivalent sequence dealing with digit-"1" pairs instead of "4"
%A _Eric Angelini_ and _Jean-Marc Falcoz_, Jun 02 2016