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 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 (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 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

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