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A096004 Number of convex triangular polyominoes containing n cells. 3
1, 1, 1, 2, 1, 2, 2, 3, 2, 2, 2, 3, 2, 3, 3, 5, 2, 3, 3, 4, 2, 4, 4, 6, 3, 3, 4, 5, 2, 5, 5, 7, 3, 4, 5, 6, 3, 5, 5, 8, 3, 4, 5, 6, 4, 7, 7, 9, 4, 5, 5, 7, 3, 7, 8, 9, 3, 5, 7, 8, 4, 8, 8, 11, 4, 5, 7, 8, 4, 9, 9, 11, 5, 5, 8, 9, 4, 9, 9, 13, 5, 7, 9, 8, 5, 8, 9, 12 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

The main sequence on triangular polyominoes is A000577. The convexity condition makes enumeration easy as a convex triangular polyomino has at most 6 sides. It is simple to prove that a(n) is also the number of 4-tuples (p,b,c,d) of nonnegative integers satisfying b<=c<=d, b+c+d<=p, n=p^2-b^2-c^2-d^2.

For n = A014529(k) there are a(n) many polygons. At least one of them can be tiled with k equilateral triangles. - Rainer Rosenthal, Sep 20 2017

LINKS

Rainer Rosenthal, Table of n, a(n) for n = 1..5200

Peter Kagey, Examples for a(1)-a(30).

FORMULA

a(n) >= sqrt(n)/3. - Baohua Tian, Apr 21 2020

EXAMPLE

a(8)=3 because there are 3 ways to compose a convex polygon of 8 equilateral triangles with side 1:

.

    *---*---*---*---*

   / \ / \ / \ / \ /

  *---*---*---*---*

      *---*---*

     / \ / \ /

    *---*---*

   / \ / \ /

  *---*---*

      *---*

     / \ / \

    *---*---*

   / \ / \ / \

  *---*---*---*

MAPLE

a:=proc(n) local x, p, d, c, b; x:=0; for p from 0 to ceil((n+1)/2) do; for d from 0 to p do; for c from 0 to min(d, p-d) do; for b from 0 to min(c, p-c-d) do; if p^2-b^2-c^2-d^2=n then x:=x+1 fi; od; od; od; od; x; end; # corrected by Rainer Rosenthal, Sep 20 2017

CROSSREFS

Cf. A000577, A014529.

Sequence in context: A263922 A057526 A033265 * A193495 A071068 A240872

Adjacent sequences:  A096001 A096002 A096003 * A096005 A096006 A096007

KEYWORD

easy,nonn

AUTHOR

Paul Boddington, Jul 27 2004

EXTENSIONS

a(83) and a(84) corrected by Rainer Rosenthal, Sep 20 2017

STATUS

approved

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Last modified May 11 03:49 EDT 2021. Contains 343784 sequences. (Running on oeis4.)