login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A049221 Number of horizontally convex n-ominoes in which the top row has exactly 1 square, which is not above the rightmost square in the second row. 2
1, 0, 1, 4, 14, 46, 148, 474, 1518, 4864, 15590, 49974, 160196, 513522, 1646134, 5276800, 16915150, 54222686, 173814580, 557174698, 1786062174, 5725346304, 18352995094, 58831800038, 188589419748, 604536478850, 1937883656166 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

LINKS

Table of n, a(n) for n=1..27.

Dean Hickerson, Counting Horizontally Convex Polyominoes, J. Integer Sequences, Vol. 2 (1999), #99.1.8.

Index entries for linear recurrences with constant coefficients, signature (5,-7,4)

FORMULA

G.f.: x (1-x)^2 (1-3x+x^2)/(1-5x+7x^2-4x^3).

a(n) = 5a(n-1) - 7a(n-2) + 4a(n-3) for n >= 6.

a(n) = A049219(n-1) + A049222(n) for n >= 3.

MATHEMATICA

a[ n_ ] := a[ n ]=If[ n<6, {1, 0, 1, 4, 14}[ [ n ] ], 5a[ n-1 ]-7a[ n-2 ]+4a[ n-3 ] ]

LinearRecurrence[{5, -7, 4}, {1, 0, 1, 4, 14}, 40] (* Harvey P. Dale, Nov 16 2015 *)

CROSSREFS

Cf. A049219, A049222.

Sequence in context: A030267 A026290 A027649 * A081670 A297016 A258255

Adjacent sequences:  A049218 A049219 A049220 * A049222 A049223 A049224

KEYWORD

nonn,easy

AUTHOR

Dean Hickerson, Aug 10 1999

STATUS

approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified September 20 12:42 EDT 2018. Contains 315239 sequences. (Running on oeis4.)