OFFSET
1,3
COMMENTS
This sequence counts only polyominoes that are strictly 2-dimensional - it excludes those where all the squares are in a single line. Thus for n > 1, a(n) = A001168(n) - 2. - Franklin T. Adams-Watters, Jul 29 2007
REFERENCES
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
Jean-François Alcover, Table of n, a(n) for n = 1..56
W. F. Lunnon, Counting multidimensional polyominoes, Computer Journal 18 (1975), no. 4, pp. 366-367.
FORMULA
a(n) = A001168(n) - 2 for n > 1. - Franklin T. Adams-Watters, Jul 29 2007
MATHEMATICA
a[n_] := If[n == 1, 0, A001168[[n]] - 2];
Array[a, 56] (* Jean-François Alcover, Sep 06 2019 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
EXTENSIONS
Name clarified and a(18)-a(28) from Andrew Howroyd, Dec 04 2018
STATUS
approved