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A320486
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Keep just the digits of n that appear exactly once; write 0 if all digits disappear.
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20
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0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 0, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 0, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 0, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 0, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 0, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 0, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 0, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 0, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 0, 1, 0, 102, 103, 104, 105, 106, 107, 108, 109, 0, 0, 2, 3, 4, 5, 6, 7, 8, 9, 120, 2, 1, 123, 124, 125, 126, 127, 128, 129, 130, 3
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OFFSET
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0,3
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COMMENTS
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Digits that appear more than once in n are erased. Leading zeros are erased unless the result is 0. If all digits are erased, we write 0 for the result (A320485 is another version, which uses -1 for the empty string).
More than the usual number of terms are shown in order to reach some interesting examples.
The number of d-digit numbers n for which a(n) > 0 is at most d*9^d, so in this sense most a(n) are 0. - Robert Israel, Oct 24 2018
The set of numbers with the property that their digits appear at least twice is of asymptotic density 1 (and the set of numbers that have a digit that occurs only once is of density 0), so in that sense it is rather exceptional for large n to have a(n) > 0. - M. F. Hasler, Oct 24 2018
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REFERENCES
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Eric Angelini, Posting to Sequence Fans Mailing List, Oct 24 2018
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LINKS
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N. J. A. Sloane, Coordination Sequences, Planing Numbers, and Other Recent Sequences (II), Experimental Mathematics Seminar, Rutgers University, Jan 31 2019, Part I, Part 2, Slides. (Mentions this sequence)
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EXAMPLE
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1231 becomes 23, 1123 becomes 23, 11231 becomes 23, and 11023 becomes 23 (as we don't accept leading zeros). Note that 112323 disappears immediately and we get 0.
101, 110, 11000, 10001 all become 0.
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MAPLE
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f:= proc(n) local F, S;
F:= convert(n, base, 10);
S:= select(t -> numboccur(t, F)>1, [$0..9]);
if S = {} then return n fi;
F:= subs(seq(s=NULL, s=S), F);
add(F[i]*10^(i-1), i=1..nops(F))
end proc:
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MATHEMATICA
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Table[If[(c=Select[b=IntegerDigits[n], Count[b, #]==1&])=={}, 0, FromDigits@c], {n, 0, 131}] (* Giorgos Kalogeropoulos, May 09 2021 *)
d1[n_]:=Module[{idn=IntegerDigits[n]}, FromDigits[If[DigitCount[n, 10, #]>1, Nothing, #]&/@ idn]]; Array[d1, 150, 0] (* Harvey P. Dale, Jun 23 2023 *)
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PROG
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(PARI) a(n) = {my(d=digits(n), v = vector(10), res = 0); for(i=1, #d, v[d[i]+1]++); for(i=1, #d, if(v[d[i]+1]==1, res=10*res+d[i])); res}
(PARI) A320486(n, D=digits(n))=fromdigits(select(d->#select(t->t==d, D)<2, D)) \\ M. F. Hasler, Oct 24 2018
(Python)
return int('0'+''.join(d if str(n).count(d) == 1 else '' for d in str(n))) # Chai Wah Wu, Nov 19 2018
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CROSSREFS
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See A320485 for a different version.
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KEYWORD
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AUTHOR
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STATUS
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approved
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