

A171899


Backwards Van Eck transform of A000002.


3



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
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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]=nm; otherwise b[n]=0.
The forwards Van Eck transform of A000002 is A078929.


LINKS

Table of n, a(n) for n=1..105.


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 n1 by 1 to 1 do
if (a[i]=a[n]) then m:=ni; break; fi
od:
b:=[op(b), m];
od:
RETURN(b);
end:


CROSSREFS

Cf. A000002, A181391, A171898, A078929.
Sequence in context: A271617 A057741 A133571 * A213885 A083208 A126682
Adjacent sequences: A171896 A171897 A171898 * A171900 A171901 A171902


KEYWORD

nonn


AUTHOR

N. J. A. Sloane, Oct 22 2010


STATUS

approved



