The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Please make a donation to keep the OEIS running. We are now in our 56th year. In the past year we added 10000 new sequences and reached almost 9000 citations (which often say "discovered thanks to the OEIS"). Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A171898 Forward van Eck transform of A181391. 10
 1, 2, 6, 2, 2, 5, 1, 6, 42, 5, 2, 4, 5, 9, 14, 3, 9, 3, 15, 2, 4, 6, 17, 3, 6, 32, 56, 5, 3, 131, 5, 11, 5, 3, 20, 6, 2, 8, 15, 31, 170, 3, 31, 18, 3, 3, 33, 5, 1, 11, 46, 56, 4, 37, 152, 307, 3, 7, 92, 4, 7, 62, 52, 3, 42, 3, 6, 2, 19, 6, 8, 3, 9, 3, 650, 2, 23, 8, 223, 7, 206, 3, 21, 25, 5, 8 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Given a sequence a, the forward van Eck transform b is defined as follows: If a(n) also appears again in a in a later position, let a(m) be the next occurrence, and set b(n)=m-n; otherwise b(n)=0. This is a permutation of the positive terms in A181391, where each term m > 0 from that sequence is shifted backwards m+1 positions. - Jan Ritsema van Eck, Aug 16 2019 LINKS Alois P. Heinz, Table of n, a(n) for n = 1..10000 FORMULA From Jan Ritsema van Eck, Aug 16 2019: (Start) A181391(i+a(i)+1) = a(i) for any i, a(i)>0. Conversely, a(j-A(181391(j)-1) = A181391(j) for any j, A181391(j)>0. (End) MAPLE ECKf:=proc(a) local b, i, m, n; if whattype(a) <> list then RETURN([]); fi: b:=[]; for n from 1 to nops(a)-1 do # does a(n) appear again? m:=0; for i from n+1 to nops(a) do if (a[i]=a[n]) then m:=i-n; break; fi od: b:=[op(b), m]; od: b:=[op(b), 0]; RETURN(b); end: MATHEMATICA terms = 100; m = 14 terms; (* Increase m until no zero appears in the output *) ClearAll[b, last]; b[_] = 0; last[_] = -1; last[0] = 2; nxt = 1; Do[hist = last[nxt]; b[n] = nxt; last[nxt] = n; nxt = 0; If[hist > 0, nxt = n - hist], {n, 3, m}]; A181391 = Array[b, m]; ECKf[a_List] := Module[{b = {}, i, m, n}, For[n = 1, n <= Length[a]-1, n++, m = 0; For[i = n+1, i <= Length[a], i++, If[a[[i]] == a[[n]], m = i-n; Break[]]]; b = Append[b, m]]; b = Append[b, 0]; Return[b]]; ECKf[A181391][[;; terms]] (* Jean-François Alcover, Oct 30 2020, after Maple *) CROSSREFS Cf. A181391 (van Eck's sequence), A171899, A171942. Sequence in context: A289382 A062539 A064136 * A330541 A320575 A110218 Adjacent sequences:  A171895 A171896 A171897 * A171899 A171900 A171901 KEYWORD nonn AUTHOR N. J. A. Sloane, Oct 22 2010 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 2 14:04 EST 2020. Contains 338877 sequences. (Running on oeis4.)