%I #6 Jul 28 2015 12:44:45
%S 1,10,100,1000,10000,100000,1000000,10000000,100000000,1000000000,21,
%T 120,120,1020,10020,100020,1000020,10000020,100000020,1000000020,201,
%U 210,1200,1200,10200,100200,1000200,10000200,100000200,1000000200
%N Concatenate the positions of digits 9, 8,..., 0 in the decimal representation of n, using 1 for the rightmost digit etc., and 0 when the digit does not occur.
%e Consider n=1103, digits '9' through '4' do not occur, digit '3' at 1st position (looking from the right), digit '2' does not occur, digit '1' in 3rd and 4th place, and digit '0' is at 2nd position. Thus a(1103) = 00000010342 = 10342.
%o (PARI) A260521=a(n)={my(p=vector(10));for(i=1,#n=if(n,Vecrev(digits(n)),[0]),p[n[i]+1]=if(p[n[i]+1],concat(p[n[i]+1],i),i));p=concat(Vecrev(p));p[1]=Str(p[1]);eval(concat(p))}
%Y Cf. A045918 (the classical "look and say" sequence).
%Y Cf. A260519 - A260522 for variants (counting from the right or starting with digit 0).
%K nonn,base
%O 0,2
%A _M. F. Hasler_, Jul 28 2015