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 A124838 Denominators of third-order harmonic numbers (defined by Conway and Guy, 1996). 4
 1, 2, 6, 4, 20, 10, 70, 56, 504, 420, 4620, 3960, 3432, 6006, 90090, 80080, 1361360, 408408, 369512, 67184, 470288, 1293292, 29745716, 27457584, 228813200, 212469400, 5736673800, 5354228880, 155272637520, 291136195350, 273491577450 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numerators are A124837. All fractions reduced. Thanks to Jonathan Sondow for verifying these calculations. He suggests that the equivalent definition in terms of first order harmonic numbers may be computationally simpler. We are happy with the description of A027612 Numerator of 1/n + 2/(n-1) + 3/(n-2) +...+ (n-1)/2 + n, but baffled by the description of A027611. REFERENCES J. H. Conway and R. K. Guy, The Book of Numbers, New York: Springer-Verlag, pp. 143 and 258-259, 1996. LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..1000 Eric Weisstein's World of Mathematics, Harmonic Number. See equation for third-order harmonic numbers. FORMULA A124837(n)/A124838(n) = Sum_{i=1..n} A027612(n)/A027611(n+1). a(n) = denominator(Sum_{m=1..n} Sum_{L=1..m} Sum_{k=1..L} 1/k). a(n) = denominator(((n+2)!/(2!*n!)) * Sum_{k=3..n+2} 1/k). - Alexander Adamchuk, Nov 11 2006 a(n) = A213999(n+2,n-1). - Reinhard Zumkeller, Jul 03 2012 EXAMPLE a(1) = 1 = denominator of 1/1. a(2) = 2 = denominator of 1/1 + 5/2 = 7/2. a(3) = 6 = denominator of 7/2 + 13/3 = 47/6. a(4) = 4 = denominator of 47/6 + 77/12 = 57/4. a(5) = 20 = denominator of 57/4 + 87/10 = 549/20. a(6) = 10 = denominator of 549/20 + 223/20 = 341/10 a(7) = 70 = denominator of 341/10 + 481/35 = 3349/70. a(8) = 1260 = denominator of 3349/70 + 4609/280 = 88327/1260. a(9) = 45 = denominator of 88327/1260 + 4861/252 = 3844/45. a(10) = 504 = denominator of 3844/45 + 55991/2520 = 54251/504, or, untelescoping: a(10) = 504 = denominator of 1/1 + 5/2 + 13/3 + 77/12 + 87/10 + 223/20 + 481/35 + 4609/252 + 4861/252 + 55991/2520 = 54251/504. MATHEMATICA Table[Denominator[(n+2)!/2!/n!*Sum[1/k, {k, 3, n+2}]], {n, 1, 40}] (* Alexander Adamchuk, Nov 11 2006 *) PROG (Haskell) a124838 n = a213999 (n + 2) (n - 1) -- Reinhard Zumkeller, Jul 03 2012 CROSSREFS Cf. A027611, A027612, A124837. Sequence in context: A009262 A127699 A220769 * A247578 A088659 A299822 Adjacent sequences:  A124835 A124836 A124837 * A124839 A124840 A124841 KEYWORD easy,frac,nonn AUTHOR Jonathan Vos Post, Nov 10 2006 EXTENSIONS Corrected and extended by Alexander Adamchuk, Nov 11 2006 STATUS approved

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Last modified July 25 23:39 EDT 2021. Contains 346294 sequences. (Running on oeis4.)