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%I #13 Mar 04 2018 19:48:32
%S 1,8,11,18,21,28,31,38,41,48,42,7,2,17,12,27,22,37,32,47,43,6,3,16,13,
%T 26,23,36,33,46,44,5,4,15,14,25,24,35,34,45,49,10,9,20,19,30,29,40,39,
%U 50,59,60,69,70,79,80,89,90,99,91,58,51,68,61,78,71,88,81,98,92,57,52,67,62,77,72,87,82,97,93,56,53,66,63,76,73,86,83,96,94,55
%N Lexicographic first sequence of positive integers such that a(n) + a(n+1) has a digit 9, and no term occurs twice.
%C A permutation of the positive integers.
%C It happens that from a(50) = 50 on, this sequence coincides with the variant A299969 (which starts at 0 and has nonnegative terms). Indeed the two sequences have the property that the terms a(1..49) resp. A299969(0..49) exactly contain all numbers from 1 to 49, respectively 0 to 49. - _M. F. Hasler_, Feb 28 2018
%H Vincenzo Librandi, <a href="/A299979/b299979.txt">Table of n, a(n) for n = 1..2000</a>
%t Nest[Append[#, Block[{k = 1}, While[Nand[FreeQ[#, k], DigitCount[#[[-1]] + k, 10, 9] > 0], k++]; k]] &, {1}, 90] (* _Michael De Vlieger_, Mar 01 2018 *)
%o (PARI) a(n,f=1,d=9,a=1,u=[a])={for(n=2,n,f&&if(f==1,print1(a","),write(f,n-1," "a));for(k=u[1]+1,oo,setsearch(u,k)&&next;setsearch(Set(digits(a+k)),d)&&(a=k)&&break);u=setunion(u,[a]);u[2]==u[1]+1&&u=u[^1]);a}
%Y Cf. A299969 (analog with nonnegative terms), A299957 (analog with digit 1), A299971, A299972, ..., A299978 (digit 0, 2, ..., 8).
%K nonn,base
%O 1,2
%A _M. F. Hasler_ and _Eric Angelini_, Feb 22 2018
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