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 A338921 a(0)=1, a(n) for n >= 1 is the number of distinct sums of two elements in [a(0), ..., a(n-1)], chosen without replacement. 0
 1, 0, 1, 2, 3, 5, 8, 12, 17, 22, 28, 35, 43, 52, 60, 69, 77, 86, 92, 103, 112, 123, 137, 151, 168, 180, 194, 204, 224, 245, 261, 280, 301, 318, 335, 352, 369, 387, 413, 433, 459, 482, 507, 528, 552, 586, 614, 638, 669, 701, 733, 761, 791, 824, 855, 885, 917, 952, 985, 1020 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS a(n) <= A000217(n)-n for n >= 1. Without replacement means a(i)+a(i) is not included. However, if a(i)=a(j), a(i)+a(j) still counts because they have two different indices. If you include a(i)+a(i), the sequence becomes A000012 (all ones). If you include the distinct sums between 3 elements and more, you arrive at the sequence 1, 0, followed by A000079 (2^n). Same rule as in A247184, but with a(0)=1. LINKS EXAMPLE a(1) gives the number of distinct sums between two elements of [1]. There aren't two elements so a(1)=0. a(2) gives the number of distinct sums between two elements of [1,0]. The only sum are 1+0, so a(2) = 1. a(3) gives the number of distinct sums between two elements of [1,0,1]. The two sums are 1+0 and 1+1 so a(3)=2. MAPLE s:= proc(n) option remember; `if`(n=0, {},       {s(n-1)[], seq(a(i)+a(n), i=0..n-1)})     end: a:= proc(n) option remember;       `if`(n=0, 1, nops(s(n-1)))     end: seq(a(n), n=0..60);  # Alois P. Heinz, Nov 16 2020 MATHEMATICA a[0] = 1; a[1] = 0; a[n_Integer?Positive] := a[n] = Length[Union[Total[Subsets[Array[a, n, 0], {2}], {2}]]]; Array[a, 61, 0] (* Jan Mangaldan, Nov 23 2020 *) PROG (PARI) my(v=[1], w=[], n=1); while(n<75, for(i=2, #v, w=concat(w, v[i-1]+v[#v])); w=vecsort(w, , 8); v=concat(v, #w); n++); v CROSSREFS Cf. A000217, A000012, A247184, A247185. Sequence in context: A028955 A246321 A104664 * A333343 A022856 A089071 Adjacent sequences:  A338918 A338919 A338920 * A338922 A338923 A338924 KEYWORD nonn AUTHOR Derek Orr, Nov 15 2020 STATUS approved

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Last modified July 26 15:49 EDT 2021. Contains 346294 sequences. (Running on oeis4.)