A171899 Backwards van Eck transform of A000002.

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

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

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 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

Cf. A000002, A181391, A171898, A078929.

Sequence in context: A133571 A357059 A326420 * A355784 A213885 A347739

Adjacent sequences: A171896 A171897 A171898 * A171900 A171901 A171902

Keyword

nonn

Author

N. J. A. Sloane, Oct 22 2010

Status

approved