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 A103217 Hexagonal numbers triangle read by rows: T(n,k)=(n+1-k)*(2*(n+1-k)-1). 1
 1, 6, 1, 15, 6, 1, 28, 15, 6, 1, 45, 28, 15, 6, 1, 66, 45, 28, 15, 6, 1, 91, 66, 45, 28, 15, 6, 1, 120, 91, 66, 45, 28, 15, 6, 1, 153, 120, 91, 66, 45, 28, 15, 6, 1, 190, 153, 120, 91, 66, 45, 28, 15, 6, 1, 231, 190, 153, 120, 91, 66, 45, 28, 15, 6, 1, 276, 231, 190, 153, 120, 91, 66 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS The triangle is generated by the product A*B = B*A of the infinite lower triangular matrices A = 1 0 0 0... 1 1 0 0... 1 1 1 0... 1 1 1 1... ... and B = 1 0 0 0... 5 1 0 0... 9 5 1 0... 13 9 5 1... ... The only prime hexagonal pyramidal number is 7. The only semiprime hexagonal pyramidal numbers are: 22, 95, 161. All greater hexagonal pyramidal numbers A002412 have at least 3 prime factors. Note that 7337 = 11 * 23 * 29 is a palindromic 3-brilliant number and 65941 = 23 * 47 * 61 is 3-brilliant. - Jonathan Vos Post, Jan 26 2005 LINKS Table of n, a(n) for n=0..72. Jonathan Vos Post, Table of Polytope Numbers, Sorted, Through 1,000,000. Eric Weisstein's World of Mathematics, Hexagonal Number. Eric Weisstein's World of Mathematics, Hexagonal Pyramidal Number. EXAMPLE Triangle begins: 1, 6,1, 15,6,1, 28,15,6,1, 45,28,15,6,1, 66,45,28,15,6,1, 91,66,45,28,15,6,1, MATHEMATICA T[n_, k_] := (n + 1 - k)*(2*(n + 1 - k) - 1); Flatten[ Table[ T[n, k], {n, 0, 10}, {k, 0, n}]] (* Robert G. Wilson v, Feb 10 2005 *) PROG (PARI) T(n, k) = (n+1-k)*(2*(n+1-k)-1); for(i=0, 10, for(j=0, i, print1(T(i, j), ", ")); print()) CROSSREFS Row sums give A002412 (hexagonal pyramidal numbers). Cf. A000384, A002412. Sequence in context: A146997 A147483 A050309 * A136273 A125233 A139727 Adjacent sequences: A103214 A103215 A103216 * A103218 A103219 A103220 KEYWORD easy,nonn,tabl AUTHOR Lambert Klasen (lambert.klasen(AT)gmx.de) and Gary W. Adamson, Jan 25 2005 STATUS approved

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