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A171899
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Backwards van Eck transform of A000002.
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4
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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)
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OFFSET
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1,4
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COMMENTS
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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.
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LINKS
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Table of n, a(n) for n=1..105.
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MAPLE
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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:
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CROSSREFS
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Cf. A000002, A181391, A171898, A078929.
Sequence in context: A133571 A357059 A326420 * A355784 A213885 A347739
Adjacent sequences: A171896 A171897 A171898 * A171900 A171901 A171902
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KEYWORD
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nonn
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AUTHOR
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N. J. A. Sloane, Oct 22 2010
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STATUS
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approved
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