OFFSET
1,2
COMMENTS
MATHEMATICA
a[n_ /; n <= 4] := 2^(n - 1); a[n_] := a[n] = (t = Table[a[i] + a[j] + a[k] + a[l], {i, 1, n - 4}, {j, i + 1, n - 3}, {k, j + 1, n - 2}, {l, k + 1, n - 1}] // Flatten; Complement[ Select[t // Tally, #[[2]] == 1 &][[All, 1]], Array[a, n - 1]] // Sort // First); Array[a, 55] (* after Jean-François Alcover A007086 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Robert G. Wilson v, Dec 20 2016
STATUS
approved