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Concatenate the positions of digits 9, 8,..., 0 in the decimal representation of n, using 1 for the leftmost digit etc., and 0 when the digit does not occur.
9

%I #5 Jul 28 2015 12:44:57

%S 1,10,100,1000,10000,100000,1000000,10000000,100000000,1000000000,12,

%T 120,210,2010,20010,200010,2000010,20000010,200000010,2000000010,102,

%U 120,1200,2100,20100,200100,2000100,20000100,200000100,2000000100

%N Concatenate the positions of digits 9, 8,..., 0 in the decimal representation of n, using 1 for the leftmost digit etc., and 0 when the digit does not occur.

%e Consider n=1103, digits '9' through '4' do not occur, digit '3' at 4th position, digit '2' does not occur, digit '1' in 1st and 2nd place, and digit '0' is at 3rd position. Thus a(1103) = 00000040123 = 40123.

%o (PARI) a(n)={my(p=vector(10));for(i=1,#n=if(n,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 - A260521 for variants (counting from the right or starting with digit 0).

%K nonn,base

%O 0,2

%A _M. F. Hasler_, Jul 28 2015