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 A124837 Numerators of third order harmonic numbers (defined by Conway and Guy, 1996). 9
 1, 7, 47, 57, 459, 341, 3349, 3601, 42131, 44441, 605453, 631193, 655217, 1355479, 23763863, 24444543, 476698557, 162779395, 166474515, 34000335, 265842403, 812400067, 20666950267, 21010170067, 192066102203, 194934439103 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Denominators are A124838. 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. a(n) are the numerators of third order harmonic numbers H(n, (3)) = Sum[ Sum[ HarmonicNumber[k], {k, 1, m}], {m, 1, n} ]. Denominators are listed in A124838(n) = {1, 2, 6, 4, 20, 10, 70, 56, 504, 420, 4620, 3960, 3432, 6006, 90090, 80080, 1361360, 408408, ...}. p divides a(p-5) for prime p>5. Primes in a(n) are listed in A129880(n) = {7, 47, 42131, 23763863, 192066102203, 5733412167187, 34745876421709, ...}. Numbers n such that a(n) is prime are listed in A129881(n) = {2, 3, 9, 15, 25, 27, 33, 45, 55, 67, 70, 93, 94, 97, 112, 113, 125, ...}. - Alexander Adamchuk, Nov 11 2006 a(n) = A213998(n+2,n-1). - Reinhard Zumkeller, Jul 03 2012 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) = Numerator[ Sum[ Sum[ Sum[ 1/k, {k,1,l} ], {l,1,m} ], {m,1,n} ] ]. a(n) = Numerator[ (n+2)!/2!/n! * Sum[ 1/k, {k,3,n+2} ] ]. - Alexander Adamchuk, Nov 11 2006 a(n) = Numerator[ (n+2)(n+1)/2 * Sum[ 1/k, {k,3,n+2} ] ]. - Alexander Adamchuk, Nov 11 2006 a(n) = Numerator of sum((-1)^k*binomial(-3,k)/(n-k),k=0..n-1). [Gary Detlefs, Jul 18 2011] EXAMPLE a(1) = 1 = numerator of 1/1. a(2) = 7 = numerator of 1/1 + 5/2 = 7/2. a(3) = 47 = numerator of 7/2 + 13/3 = 47/6. a(4) = 57 = numerator of 47/6 + 77/12 = 57/4. a(5) = 549 = numerator of 57/4 + 87/10 = 549/20. a(6) = 341 = numerator of 549/20 + 223/20 = 341/10 a(7) = 3349 = numerator of 341/10 + 481/35 = 3349/70. a(8) = 88327 = numerator of 3349/70 + 4609/280 = 88327/1260. a(9) = 3844 = numerator of 88327/1260 + 4861/252 = 3844/45. a(10) = 54251 = numerator of 3844/45 + 55991/2520 = 54251/504, or, untelescoping: a(10) = 54251 = numerator 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[Numerator[(n+2)!/2!/n!*Sum[1/k, {k, 3, n+2}]], {n, 1, 30}] (* Alexander Adamchuk, Nov 11 2006 *) PROG (Haskell) a124837 n = a213998 (n + 2) (n - 1) -- Reinhard Zumkeller, Jul 03 2012 CROSSREFS Cf. A027611, A027612, A124838. Cf. A001008, A002805, A067657, A056903, A124878, A124879, A124837, A129880, A129881. Sequence in context: A086040 A009241 A263920 * A122731 A059452 A245229 Adjacent sequences:  A124834 A124835 A124836 * A124838 A124839 A124840 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 September 23 19:48 EDT 2021. Contains 347617 sequences. (Running on oeis4.)