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 A101686 a(n) = Product_{i=1..n} (i^2 + 1). 26
 1, 2, 10, 100, 1700, 44200, 1635400, 81770000, 5315050000, 435834100000, 44019244100000, 5370347780200000, 778700428129000000, 132379072781930000000, 26078677338040210000000, 5893781078397087460000000, 1514701737148051477220000000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Sum of all coefficients in Product_{k=0..n} (x + k^2). Row sums of triangle of central factorial numbers (A008955). "HANOWA" is a matrix whose eigenvalues lie on a vertical line. It is an N X N matrix with 2 X 2 blocks with identity matrices in the upper left and lower right blocks and diagonal matrices containing the first N integers in the upper right and lower left blocks. In MATLAB, the following code generates the sequence... for n=0:2:TERMS*2 det(gallery('hanowa',n)) end. - Paul Max Payton, Mar 31 2005 Cilleruelo shows that a(n) is a square only for n = 0 and 3. - Charles R Greathouse IV, Aug 27 2008 a(n) = A231530(n)^2 + A231531(n)^2. - Stanislav Sykora, Nov 10 2013 REFERENCES Edmund Landau, Handbuch der Lehre von der Verteilung der Primzahlen, Chelsea Publishing, NY 1953, pp. 559-561, Section 147. - N. J. A. Sloane, May 29 2014 LINKS Stanislav Sykora, Table of n, a(n) for n = 0..252 Javier Cilleruelo, Squares in (1^2+1)...(n^2+1), Journal of Number Theory 128:8 (2008), pp. 2488-2491. Erhan Gürela, Ali Ulas Özgür Kisisel, A note on the products (1^mu + 1)(2^mu + 1)···(n^mu + 1), Journal of Number Theory, Volume 130, Issue 1, January 2010, Pages 187-191. V. H. Moll, An arithmetic conjecture on a sequence of arctangent sums, 2012. FORMULA G.f.: 1/(1-x) = Sum_{n>=0} a(n)*x^n / Product_{k=1..n+1} (1 + k^2*x). - Paul D. Hanna, Jan 07 2013 G.f.:1 + x*(G(0) - 1)/(x-1) where G(k) =  1 - ((k+1)^2+1)/(1-x/(x - 1/G(k+1) )); (continued fraction). - Sergei N. Gladkovskii, Jan 15 2013. a(n) ~ (n!)^2 * sinh(Pi)/Pi. - Vaclav Kotesovec, Nov 11 2013 From Vladimir Reshetnikov, Oct 25 2015: (Start) a(n) = Gamma(n+1+i)*Gamma(n+1-i)*sinh(Pi)/Pi. a(n) ~ 2*exp(-2*n)*n^(2*n+1)*sinh(Pi). G.f. for 1/a(n): hypergeom([1], [1-i, 1+i], x). E.g.f. for a(n)/n!: hypergeom([1-i, 1+i], [1], x), where i=sqrt(-1). Recurrence: a(0) = 1, a(n) = (n^2+1)*a(n-1). (End) a(n+3)/a(n+2) - 2 a(n+2)/a(n+1) + a(n+1)/a(n) = 2. - Robert Israel, Oct 25 2015 a(n) = A003703(n+1)^2 + A009454(n+1)^2. - Vladimir Reshetnikov, Oct 15 2016 MAPLE with(combinat): a := n -> mul(fibonacci(3, i), i=0..n): seq(a(n), n=0..14); # Zerinvary Lajos, Apr 25 2008 p := n -> mul(x^2+1, x=0..n): seq(p(i), i=0..14); # Gary Detlefs, Jun 03 2010 MATHEMATICA Table[Product[k^2+1, {k, 0, n}], {n, 0, 20}] (* Vaclav Kotesovec, Nov 11 2013 *) Table[Pochhammer[I, n + 1] Pochhammer[-I, n + 1], {n, 0, 20}] (* Vladimir Reshetnikov, Oct 25 2015 *) Table[Abs[Pochhammer[1 + I, n]]^2, {n, 0, 20}] (* Vaclav Kotesovec, Oct 16 2016 *) PROG (PARI) a(n)=prod(k=1, n, k^2+1) \\ Charles R Greathouse IV, Aug 27 2008 (PARI) {a(n)=if(n==0, 1, 1-polcoeff(sum(k=0, n-1, a(k)*x^k/prod(j=1, k+1, 1+j^2*x+x*O(x^n))), n))} \\ Paul D. Hanna, Jan 07 2013 CROSSREFS Equals 2 * A051893(n+1), n>0. Cf. A156648. Cf. A255433, A255434, A255435, A003703, A009454. Sequence in context: A277468 A099826 A063959 * A188193 A228120 A074109 Adjacent sequences:  A101683 A101684 A101685 * A101687 A101688 A101689 KEYWORD nonn AUTHOR Ralf Stephan, Dec 13 2004 EXTENSIONS More terms from Charles R Greathouse IV, Aug 27 2008 Simpler definition from Gary Detlefs, Jun 03 2010 Entry revised by N. J. A. Sloane, Dec 22 2012 Minor edits by Vaclav Kotesovec, Mar 13 2015 STATUS approved

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Last modified April 19 11:04 EDT 2019. Contains 322260 sequences. (Running on oeis4.)