A118907 a(0) = 1; a(n) = |a(n-1) - n| if n is has already appeared in the sequence, otherwise a(n) = a(n-1) + n.

1, 0, 2, 5, 9, 4, 10, 17, 25, 16, 6, 17, 29, 42, 56, 71, 55, 38, 56, 75, 95, 116, 138, 161, 185, 160, 186, 213, 241, 212, 242, 273, 305, 338, 372, 407, 443, 480, 442, 481, 521, 562, 520, 563, 607, 652, 698, 745, 793, 842, 892, 943, 995, 1048, 1102, 1047, 991

Offset

0,3

Links

Table of n, a(n) for n=0..56.

Maple

a[0]:=1: S:={1}: for n from 1 to 65 do if member(n, S)=true then a[n]:=abs(a[n-1]-n): S:=S union {a[n]} else a[n]:=a[n-1]+n: S:=S union {a[n]} fi od: seq(a[n], n=0..65); # Emeric Deutsch, May 07 2006

Mathematica

f[s_] := Block[{n, }, n = Length[s]; Return[Append[s, Abs[Last[s] + If[MemberQ[s, n], -n, +n]]]]; ]; Nest[f, {1}, 66] (* Ray Chandler, May 06 2006 *)

Crossrefs

Cf. A005132, A118906.

Sequence in context: A021798 A336830 A065226 * A246206 A200336 A065225

Adjacent sequences: A118904 A118905 A118906 * A118908 A118909 A118910

Keyword

easy,nonn

Author

Leroy Quet, May 05 2006

Extensions

Extended by Ray Chandler and Emeric Deutsch, May 06 2006

Status

approved