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A171899 Backwards van Eck transform of A000002. 4
0, 0, 1, 3, 1, 3, 2, 2, 1, 3, 2, 1, 3, 1, 3, 2, 1, 3, 1, 3, 2, 2, 1, 3, 2, 2, 1, 3, 1, 3, 2, 1, 3, 2, 2, 1, 3, 2, 1, 3, 1, 3, 2, 2, 1, 3, 2, 2, 1, 3, 2, 1, 3, 1, 3, 2, 1, 3, 1, 3, 2, 2, 1, 3, 2, 1, 3, 1, 3, 2, 1, 3, 2, 2, 1, 3, 2, 2, 1, 3, 1, 3, 2, 1, 3, 1, 3, 2, 2, 1, 3, 2, 1, 3, 1, 3, 2, 1, 3, 1, 3, 2, 2, 1, 3 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,4
COMMENTS
Given a sequence a, the backwards van Eck transform b is defined as follows: If a(n) has already appeared in a, let a(m) be the most recent occurrence, and set b(n)=n-m; otherwise b(n)=0.
The forwards van Eck transform of A000002 is A078929.
LINKS
MAPLE
ECKb:=proc(a) local b, i, m, n;
if whattype(a) <> list then RETURN([]); fi:
b:=[0];
for n from 2 to nops(a) do
# has a(n) appeared before?
m:=0;
for i from n-1 by -1 to 1 do
if (a[i]=a[n]) then m:=n-i; break; fi
od:
b:=[op(b), m];
od:
RETURN(b);
end:
CROSSREFS
Sequence in context: A133571 A357059 A326420 * A355784 A213885 A347739
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Oct 22 2010
STATUS
approved

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Last modified March 19 07:25 EDT 2024. Contains 370955 sequences. (Running on oeis4.)