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A057167 Term in Recamán's sequence A005132 where n appears for first time, or -1 if n never appears. 21
0, 1, 4, 2, 131, 129, 3, 5, 16, 14, 12, 10, 8, 6, 31, 29, 27, 25, 23, 99734, 7, 9, 11, 13, 15, 17, 64, 62, 60, 58, 56, 54, 52, 50, 48, 46, 44, 42, 40, 38, 111, 22, 20, 18, 28, 30, 32, 222, 220, 218, 216, 214, 212, 210, 208, 206, 204, 202, 200, 198, 196 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,3
LINKS
T. D. Noe, Table of n, a(n) for n=0..1642 (taking a(1355) from A064227)
N. J. A. Sloane, My favorite integer sequences, in Sequences and their Applications (Proceedings of SETA '98).
MAPLE
w := array(1..10000); for j from 1 to 100 do l := 0; for k from 1 to nops(a) do if a[k] = j then l := k; exit; fi; od: w[j] := l; od: s := [seq(w[j], j=1..100)]; # where a is an array formed from sequence A005132
MATHEMATICA
A005132 = {0}; Do[If[(r = Last[A005132] - n) <= 0 || MemberQ[ A005132, r], r = r + 2n]; AppendTo[ A005132, r], {n, 1, 10^5}]; a[n_] := If[p = Position[ A005132, n]; p == {}, 0, p[[1, 1]] - 1]; Table[a[n], {n, 1, 60}] (* Jean-François Alcover, Jul 18 2012 *)
PROG
(PARI) first(n) = my(a=vector(n), r=[0]); while(#Set(a)<n, my(i=#r, j=r[i]-i, k=j+(j<1 || setsearch(Set(r), j))*2*i); r=concat(r, [k]); if(k<n && !a[k+1], a[k+1]=i)); a \\ Iain Fox, Jul 11 2022
(Python)
from itertools import count, islice
def agen(): # generator of terms
an, A005132set, inv, y = 0, {0}, {0: 0}, 0
for n in count(1):
t = an - n
an = t if t >= 0 and t not in A005132set else an + n
A005132set.add(an)
inv[an] = n
while y in inv: yield inv[y]; y += 1
print(list(islice(agen(), 61))) # Michael S. Branicky, Jul 12 2022
CROSSREFS
Cf. also A187943, A187922.
Sequence in context: A201228 A010319 A266069 * A355646 A096683 A158903
KEYWORD
nonn,easy,nice,look
AUTHOR
N. J. A. Sloane, Sep 14 2000
EXTENSIONS
I conjecture a(n) is never -1 - but see A064227, A064228.
a(0)=0 added and escape clause value changed to -1 by N. J. A. Sloane, May 01 2020
STATUS
approved

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Last modified March 18 22:56 EDT 2024. Contains 370952 sequences. (Running on oeis4.)