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 A209805 Triangle read by rows: T(n,k) is the number of k-block noncrossing partitions of n-set up to rotations. 5
 1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 2, 4, 2, 1, 1, 3, 10, 10, 3, 1, 1, 3, 15, 25, 15, 3, 1, 1, 4, 26, 64, 64, 26, 4, 1, 1, 4, 38, 132, 196, 132, 38, 4, 1, 1, 5, 56, 256, 536, 536, 256, 56, 5, 1, 1, 5, 75, 450, 1260, 1764, 1260, 450, 75, 5, 1 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,8 COMMENTS Like the Narayana triangle A001263 (and unlike A152175) this triangle is symmetric. The diagonal entries are 1, 1, 4, 25, 196, 1764, ... which is probably sequence A001246 - the squares of the Catalan numbers. The above conjecture about the diagonal entries T(2*n-1, n) is true since gcd(2*n-1, n) = gcd(2*n-1, n-1) = 1 and then T(2*n-1, n) simplifies to A001246(n-1) using the formula given below. - Andrew Howroyd, Nov 15 2017 LINKS Andrew Howroyd, Table of n, a(n) for n = 1..1275 Tilman Piesk, Partition related number triangles (Wikiversity) Tilman Piesk, Brute force MATLAB code used to calculate the rows (Pastebin) FORMULA T(n,k) = (1/n)*((Sum_{d|gcd(n,k)} phi(d)*A103371(n/d-1,k/d-1)) + (Sum_{d|gcd(n,k-1)} phi(d)*A103371(n/d-1,(n+1-k)/d-1)) - A132812(n,k)). - Andrew Howroyd, Nov 15 2017 EXAMPLE Triangle begins:   1;   1,   1;   1,   1,   1;   1,   2,   2,   1;   1,   2,   4,   2,   1;   1,   3,  10,  10,   3,   1;   1,   3,  15,  25,  15,   3,   1;   1,   4,  26,  64,  64,  26,   4,   1;   1,   4,  38, 132, 196, 132,  38,   4,   1;   1,   5,  56, 256, 536, 536, 256,  56,   5,   1; MATHEMATICA b[n_, k_] := Binomial[n-1, n-k] Binomial[n, n-k]; T[n_, k_] := (DivisorSum[GCD[n, k], EulerPhi[#] b[n/#, k/#]&] + DivisorSum[GCD[n, k - 1], EulerPhi[#] b[n/#, (n + 1 - k)/#]&] - k Binomial[n, k]^2/(n - k + 1))/n; Table[T[n, k], {n, 1, 12}, {k, 1, n}] // Flatten (* Jean-François Alcover, Jul 01 2018, after Andrew Howroyd *) PROG (PARI) b(n, k)=binomial(n-1, n-k)*binomial(n, n-k); T(n, k)=(sumdiv(gcd(n, k), d, eulerphi(d)*b(n/d, k/d)) + sumdiv(gcd(n, k-1), d, eulerphi(d)*b(n/d, (n+1-k)/d)) - k*binomial(n, k)^2/(n-k+1))/n; \\ Andrew Howroyd, Nov 15 2017 CROSSREFS Cf. A054357 (row sums), A001246 (square Catalan numbers). Cf. A001263, A103371, A132812, A209612. Sequence in context: A172479 A122085 A209612 * A238453 A066287 A059260 Adjacent sequences:  A209802 A209803 A209804 * A209806 A209807 A209808 KEYWORD nonn,tabl AUTHOR Tilman Piesk, Mar 13 2012 STATUS approved

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Last modified December 9 19:51 EST 2019. Contains 329879 sequences. (Running on oeis4.)