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 A090681 Expansion of (sec(x/2)^2 + sech(x/2)^2)/2 in powers of x^4. 3
 1, 1, 31, 5461, 3202291, 4722116521, 14717667114151, 86125672563201181, 868320396104950823611, 14129659550745551130667441, 352552873457246307069012458671, 12942188000689093683411117827763301, 675618013651758631167025175564066787331, 48743995308245045290420262686473639399176761 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS Stefano Spezia, Table of n, a(n) for n = 0..135 FORMULA a(n) = -G(2n+1)/(2n+1) where G(k) is the k-th Genocchi number of first kind (A001469). a(n) = A002425(2n+1). a(n) = A012853(n)/2^(4n+1). a(n) = abs(A012670(n)/2^(6n+1)). E.g.f.: (sec(x/2)^2 + sech(x/2)^2)/2 = Sum_{k>=1} a(k)*x^(4k)/(4k)!. - Michael Somos, Mar 06 2004 a(n) == 1 (mod 30). - Michael Somos, Jul 23 2005 EXAMPLE (sec(x/2)^2 + sech(x/2)^2)/2 = 1 + x^4/4! + 31*x^8/8! + 5461*x^12/12! + ... MAPLE a := n->(2*2^(4*n+2)-2)*bernoulli(4*n+2)/(2*n+1): seq(a(n), n = 0 .. 15); # Stefano Spezia, Jun 14 2019 MATHEMATICA a[n_]:=2*(2^(4*n+2)-1)*BernoulliB[4*n+2]/(2*n+1); Array[a, 15, 0] (* Stefano Spezia, Jun 14 2019 *) PROG (PARI) a(n)=if(n<0, 0, n*=4; n!*polcoeff(1/cosh(x/2+x*O(x^n))^2+1/cos(x/2+x*O(x^n))^2, n)/2) /* Michael Somos, Mar 06 2004 */ (PARI) a(n)=if(n<0, 0, n=4*n+2; 4*(2^n-1)*bernfrac(n)/n) /* Michael Somos, Mar 06 2004 */ (MAGMA) [2*(4^(2*n+1) -1)*BernoulliNumber(4*n+2)/(2*n+1): n in [0..15]]; // G. C. Greubel, Jun 28 2019 (Sage) [2*(4^(2*n+1)-1)*bernoulli(4*n+2)/(2*n+1) for n in (0..15)] # G. C. Greubel, Jun 28 2019 CROSSREFS Cf. A001469, A002425, A012670, A012853. Sequence in context: A110848 A214109 A245290 * A297767 A065756 A263378 Adjacent sequences:  A090678 A090679 A090680 * A090682 A090683 A090684 KEYWORD nonn AUTHOR Benoit Cloitre, Dec 18 2003 STATUS approved

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Last modified January 19 04:56 EST 2022. Contains 350464 sequences. (Running on oeis4.)