The OEIS mourns the passing of Jim Simons and is grateful to the Simons Foundation for its support of research in many branches of science, including the OEIS.
The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A006801 Number of 2-dimensional directed compact animals of size n. (Formerly M1440) 1
 1, 2, 5, 13, 34, 90, 239, 635, 1689, 4494, 11960, 31832, 84727, 225524, 600302, 1597904, 4253371, 11321838, 30137079, 80220557, 213535632, 568401380, 1513003529 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Directed d- diagonally convex polyominoes. [David Bevan, Mar 15 2011] REFERENCES V. K. Bhat et al., Enumeration of directed compact site animals in two dimensions, J. Phys. A 19 (1986), 3261-3265. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Table of n, a(n) for n=1..23. EXAMPLE a(3)=5: the six fixed trominoes excluding one orientation of the L tromino. [David Bevan, Mar 15 2011] MATHEMATICA a[n_, k_]:=0/; n<1||k<1||k(k+1)/2>n; a[1, 1]=1; a[n_, k_]:=a[n, k]=Sum[(j-k+2)a[n-k, j], {j, k-1, n-k}]; a[n_]:=Sum[a[n, k], {k, n}]; Array[a, 30] (* David Bevan, Mar 15 2011 *) CROSSREFS Cf. A187276 (d- diagonally convex polyominoes). Sequence in context: A112842 A097417 A367657 * A329674 A114173 A238435 Adjacent sequences: A006798 A006799 A006800 * A006802 A006803 A006804 KEYWORD nonn AUTHOR N. J. A. Sloane. EXTENSIONS Name and offset corrected, and more terms added by David Bevan, Mar 15 2011. STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified May 28 12:54 EDT 2024. Contains 372913 sequences. (Running on oeis4.)