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%I #16 Jan 02 2023 12:30:48
%S 1,2,4,7,44,47,48,49,74,77,78,79,84,87,88,89,94,97,98,99,444,447,448,
%T 449,474,477,478,479,484,487,488,489,494,497,498,499,744,747,748,749,
%U 774,777,778,779,784,787,788,789,794,797,798,799,844,847,848,849,874,877
%N Smallest strictly increasing sequence such that no term is the sum of any two digits of the sequence.
%C "Smallest" means lexicographically (1 < 2 < 10 < ...) first.
%C From a(2)=2 on, there may not occur any other term with the digit 1. From a(3)=4 on, the digits 2 and 3 are excluded. From a(4)=7 on, the digits 5 and 6 are excluded. From a(5)=44 on, the digit 0 is also excluded, and subsequent terms are all larger numbers made from digits 4,7,8 or 9; one can check that then no further contradictions can appear.
%C Thus there are 4^d terms with d digits, for d=1,2,3,... This leads to an explicit formula for the n-th term.
%H E. Angelini, <a href="http://list.seqfan.eu/oldermail/seqfan/2013-February/010739.html">Post to the SeqFan mailing list</a>, Feb 02 2013
%o (PARI) A211186(n)={n>4 & for(d=1,n--, n < 4^d & return(sum(k=1,d,[4,7,8,9][n%4+1]*10^(k-1)+0*n\=4)); n -= 4^d); [1,2,4,7][n]}
%K nonn,base
%O 1,2
%A _Eric Angelini_ and _M. F. Hasler_, Feb 02 2013
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