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A274498
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Triangle read by rows: T(n,k) is the number of ternary words of length n having degree of asymmetry equal to k (n>=0; 0<=k<=n/2).
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3
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1, 3, 3, 6, 9, 18, 9, 36, 36, 27, 108, 108, 27, 162, 324, 216, 81, 486, 972, 648, 81, 648, 1944, 2592, 1296, 243, 1944, 5832, 7776, 3888, 243, 2430, 9720, 19440, 19440, 7776, 729, 7290, 29160, 58320, 58320, 23328, 729, 8748, 43740, 116640, 174960, 139968, 46656
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OFFSET
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0,2
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COMMENTS
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The degree of asymmetry of a finite sequence of numbers is defined to be the number of pairs of symmetrically positioned distinct entries. Example: the degree of asymmetry of (2,7,6,4,5,7,3) is 2, counting the pairs (2,3) and (6,5).
A sequence is palindromic if and only if its degree of asymmetry is 0.
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LINKS
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FORMULA
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T(n,k) = 2^k*3^ceiling(n/2)*binomial(floor(n/2),k).
G.f.: G(t,z) = (1 + 3z)/(1 - 3(1 + 2t)z^2).
The row generating polynomials P[n] satisfy P[n] = 3(1 + 2t)P[n-2] (n>=2). Easy to see if we note that the ternary words of length n (n>=2) are 0w0, 0w1, 0w2, 1w0, 1w1, 1w2, 2w0, 2w1, 2w2, where w is a ternary word of length n - 2.
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EXAMPLE
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Triangle begins:
1;
3;
3, 6;
9, 18;
9, 36, 36;
27, 108, 108;
27, 162, 324, 216;
81, 486, 972, 648;
81, 648, 1944, 2592, 1296;
...
(End)
T(2,0) = 3 because we have 00, 11, and 22.
T(2,1) = 6 because we have 01, 02, 10, 12, 20, and 21.
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MAPLE
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T := proc(n, k) options operator, arrow: 2^k*3^ceil((1/2)*n)*binomial(floor((1/2)*n), k) end proc: for n from 0 to 15 do seq(T(n, k), k = 0 .. floor((1/2)*n)) end do; # yields sequence in triangular form
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MATHEMATICA
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T[n_, k_] := 2^k 3^Ceiling[n/2] Binomial[Floor[n/2], k];
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PROG
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(PARI)
T(n, k) = 2^k*3^ceil(n/2)*binomial(floor(n/2), k);
for(n=0, 10, for(k=0, n\2, print1(T(n, k), ", ")); print); \\ Andrew Howroyd, Jan 10 2018
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CROSSREFS
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KEYWORD
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nonn,tabf
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AUTHOR
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STATUS
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approved
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