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A085691
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Triangle read by rows: T(n,k) is the number of triangles of side k in triangular matchstick arrangement of side n; n>=1 and k>=1.
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5
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1, 4, 1, 9, 3, 1, 16, 7, 3, 1, 25, 13, 6, 3, 1, 36, 21, 11, 6, 3, 1, 49, 31, 18, 10, 6, 3, 1, 64, 43, 27, 16, 10, 6, 3, 1, 81, 57, 38, 24, 15, 10, 6, 3, 1, 100, 73, 51, 34, 22, 15, 10, 6, 3, 1, 121, 91, 66, 46, 31, 21, 15, 10, 6, 3, 1, 144, 111, 83, 60, 42, 29, 21, 15, 10, 6, 3, 1
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OFFSET
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1,2
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COMMENTS
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Sub-triangles can be oriented in one of two ways. The number of sub-triangles that are oriented in the same way as the full triangle is binomial(n-k+2, 2). For k <= n/2, there are also sub-triangles oriented at 180 degrees and the number of these is binomial(n-2*k+2, 2). - Andrew Howroyd, Jan 06 2020
The matchstick arrangement consists of 3*A000217(n) matchsticks. One can also consider it as a tower of cards with n base cards. - Wolfdieter Lang, Apr 06 2020
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REFERENCES
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J. H. Conway and R. K. Guy, The Book of Numbers, Copernicus Press, NY, 1996, p. 83.
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LINKS
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FORMULA
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T(n, k) = 0 for n < k; T(n, k) = (n-k+1)*(n-k+2)/2 for k <= n < 2*k; T(n, k) = n^2 - 3*(k-1)*n + (5*k-4)*(k-1)/2 for 2*k <= n.
T(n, k) = Tup(n, k) + Tdown(n, k), with Tup(n, k) = (-1)*(n-k)*A122432(n-1, k-1) and Tdown(n, k) = A332442(n, k), for n >= 1, and k = 1, 2, ..., n. - Wolfdieter Lang, Apr 06 2020
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EXAMPLE
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Triangle begins:
1;
4, 1;
9, 3, 1;
16, 7, 3, 1;
25, 13, 6, 3, 1;
36, 21, 11, 6, 3, 1;
49, 31, 18, 10, 6, 3, 1;
64, 43, 27, 16, 10, 6, 3, 1;
81, 57, 38, 24, 15, 10, 6, 3, 1;
100, 73, 51, 34, 22, 15, 10, 6, 3, 1;
...
Row n=3: In the triangle illustrated below there are 9 small triangles, 3 triangles with side length 2 and 1 with side length 3.
o
/ \
o---o
/ \ / \
o---o---o
/ \ / \ / \
o---o---o---o
(End)
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PROG
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(PARI) T(n, k)={binomial(n-k+2, 2) + if(2*k<=n, binomial(n-2*k+2, 2), 0)} \\ Andrew Howroyd, Jan 06 2020
(PARI) T(n, k)={if(k>n, 0, if(2*k > n, (n-k+1)*(n-k+2)/2, n^2 - 3*(k-1)*n + (5*k-4)*(k-1)/2))} \\ Andrew Howroyd, Jan 06 2020
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CROSSREFS
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KEYWORD
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AUTHOR
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EXTENSIONS
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Offset corrected and terms a(37) and beyond from Andrew Howroyd, Jan 05 2020
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STATUS
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approved
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