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A046854
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Triangle in which k-th entry of row n is binomial(floor(n/2+k/2),k).
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41
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1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 2, 3, 1, 1, 1, 3, 3, 4, 1, 1, 1, 3, 6, 4, 5, 1, 1, 1, 4, 6, 10, 5, 6, 1, 1, 1, 4, 10, 10, 15, 6, 7, 1, 1, 1, 5, 10, 20, 15, 21, 7, 8, 1, 1, 1, 5, 15, 20, 35, 21, 28, 8, 9, 1, 1, 1, 6, 15, 35, 35, 56, 28, 36, 9, 10, 1, 1, 1, 6, 21
(list; table; graph; refs; listen; history; internal format)
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OFFSET
| 0,8
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COMMENTS
| Row sums are fibonacci(n+2). Diagonal sums are A016116. - Paul Barry (pbarry(AT)wit.ie), Jul 07 2004
Riordan array (1/(1-x), x/(1-x^2)). Matrix inverse is A106180. - Paul Barry (pbarry(AT)wit.ie), Apr 24 2005
As an infinite lower triangular matrix * [1,2,3,...] = A055244: (1, 1, 3, 6, 12, 23,...). [From Gary W. Adamson (qntmpkt(AT)yahoo.com), Dec 23 2008]
Contribution from Emeric Deutsch (deutsch(AT)duke.poly.edu), Jun 18 2010: (Start)
T(n,k) is the number of alternating parity increasing subsequences of {1,2,...,n} of size k, starting with an odd number (Terquem's problem, see the Riordan reference, p. 17). Example: T(8,5)=6 because we have 12345, 12347, 12367, 14567, and 34567.
T(n,k) is the number of alternating parity increasing subsequences of {1,2,...,n,n+1} of size k, starting with an even number. Example: T(7,4)=5 because we have 2345, 2347, 2367, 2567, and 4567. (End)
Contribution from L. Edson Jeffery, Mar 01 2011: (Start)
This triangle can be constructed as follows. Interlace two copies of the table of binomial coefficients to get the preliminary table
1
1
1 1
1 1
1 2 1
1 2 1
1 3 3 1
1 3 3 1
...,
then shift each entire r-th column up r rows, r=0,1,2,.... Also, a signed version of this sequence (A187660 in tabular form) begins with
{1}
{1,-1}
{1,-1,-1}
{1,-2,-1,1}
{1,-2,-3,1,1}, ...
(compare with A066170, A130777). Let T(N,k) denote the k-th entry in row N of the signed table. Then, for N>1, row N gives the coefficients of the characteristic function p_N(x)=Sum[k=0..N, T(N,k)x^(N-k)]=0 of the N X N matrix U_N=[(0 ... 0 1);(0 ... 0 1 1);...;(0 1 ... 1);(1 ... 1)]. Now let Q_r(t) be a polynomial with recurrence relation Q_r(t)=t*Q_(r-1)(t)-Q_(r-2)(t) (r>1), with Q_0(t)=1 and Q_1(t)=t. Then p_N(x)=0 has solutions Q_(N-1)(phi_j), where phi_j=2*(-1)^(j-1)*cos(j*Pi/(2*N+1)), j=1,2,...,N.
For example, row N=3 is {1,-2,-1,1}, giving the coefficients of the characteristic function p_3(x)=x^3-2*x^2-x+1=0 for the 3 X 3 matrix U_3=[(0 0 1);(0 1 1);(1 1 1)], with eigenvalues Q_2(phi_j)=[2*(-1)^(j-1)*cos(j*Pi/7)]^2-1, j=1,2,3. (End)
Given the signed polynomials (+--++--,...) of the triangle, the largest root of the n-th row polynomial is the longest (2n+1) regular polygon diagonal length, with edge = 1. Example: the largest root to x^3 - 2x^2 - x + 1 = 0 is 2.24697...; the longest heptagon diagonal, ((Sin 3*Pi/7)/Sin Pi/7)) - Gary W. Adamson, Sep 06 2011.
Given the signed polynomials from Gary W. Adamson's comment, the largest root of the n-th polynomial also equals the length from the center to a corner (vertex) of a regular 2*(2*n+1)-sided polygon with side (edge) length = 1. - L. Edson Jeffery, Jan 01 2012
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REFERENCES
| J. Riordan, An Introduction to Combinatorial Analysis, Princeton University Press, 1978. [From Emeric Deutsch (deutsch(AT)duke.poly.edu), Jun 18 2010]
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LINKS
| Nathaniel Johnston, Rows 0..100, flattened
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FORMULA
| T(n,k) = binomial(floor(n/2+k/2),k)
G.f.: (1+x)/(1-xy-x^2). - Ralf Stephan, Feb 13 2005
Triangle = A097806 * A049310, as infinite lower triangular matrices. - Gary W. Adamson (qntmpkt(AT)yahoo.com), Oct 28 2007
T(n,k) = A065941(n,n-k) = abs(A130777(n,k)) = abs(A066170(n,k)) = abs(A187660(n,k)) [Johannes W. Meijer, Aug 08 2011]
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EXAMPLE
| Triangle begins:
1
1 1
1 1 1
1 2 1 1
1 2 3 1 1
1 3 3 4 1 1
...
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MAPLE
| A046854:= proc(n, k): binomial(floor(n/2+k/2), k) end: seq(seq(A046854(n, k), k=0..n), n=0..11); # Nathaniel Johnston, Jun 30 2011
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MATHEMATICA
| Table[ Binomial[ Floor[ n/2 +k/2 ], k ], {n, 0, 16}, {k, 0, n} ]
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CROSSREFS
| Reflected version of A065941, which is considered the main entry. A deficient version is in A030111.
Cf. A066170, A097806, A049310, A187660.
Cf. A055244 [From Gary W. Adamson (qntmpkt(AT)yahoo.com), Dec 23 2008]
Sequence in context: A096670 A130461 A130777 * A187660 A066170 A184957
Adjacent sequences: A046851 A046852 A046853 * A046855 A046856 A046857
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KEYWORD
| nonn,tabl,easy
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AUTHOR
| Wouter Meeussen (wouter.meeussen(AT)pandora.be)
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