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%I #21 Apr 22 2021 22:08:46
%S 0,1,3,4,5,6,7,8,9,10,11,1,13,14,15,16,17,18,19,0,1,3,4,5,6,7,8,9,30,
%T 31,3,33,34,35,36,37,38,39,40,41,4,43,44,45,46,47,48,49,50,51,5,53,54,
%U 55,56,57,58,59,60,61,6,63,64,65,66,67,68,69,70,71,7,73,74,75
%N Delete all 2's from the sequence of nonnegative integers.
%t d[n_]:=IntegerDigits[n]; t={0}; Do[If[Union[d[n]]!={2},n=FromDigits[DeleteCases[d[n],2]]; AppendTo[t,n]],{n,75}]; t (* _Jayanta Basu_, May 17 2013 *)
%o (Python)
%o def A004721(n):
%o l = len(str(n))
%o m = 2*(10**l-1)//9
%o k = n + l - int(n+l < m)
%o return 1 if k == m else int(str(k).replace('2','')) # _Chai Wah Wu_, Apr 20 2021
%Y Cf. A004719, A004720, A004722, A004723, A004724, A004725, A004726, A004727, A004728.
%K base,nonn
%O 0,3
%A _N. J. A. Sloane_
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