OFFSET
1,2
LINKS
Robert Israel, Table of n, a(n) for n = 1..200
FORMULA
E.g.f.: L(x) = Series_Reversion( Integral sqrt( cosh(2*x) ) dx ).
(1) L(x) = log(C(x) + S(x)),
(2) cosh(2*L(x)) = C(x)^2 + S(x)^2,
(3) cosh(L(x)) = C(x) and sinh(L(x)) = S(x),
where C(x)^2 - S(x)^2 = 1 and C'(x)^2 + S'(x)^2 = 1.
EXAMPLE
E.g.f.: L(x) = x - 2*x^3/3! + 44*x^5/5! - 2840*x^7/7! + 367760*x^9/9! - 79719200*x^11/11! + 26016555200*x^13/13! - 11921650083200*x^15/15! + 7300922254496000*x^17/17! - 5759532173685056000*x^19/19! + 5688335131502291840000*x^21/21! +...
where
1/sqrt( cosh(2*L(x)) ) = 1 - 2*x^2/2! + 44*x^4/4! - 2840*x^6/6! + 367760*x^8/8! - 79719200*x^10/10! + 26016555200*x^12/12! - 11921650083200*x^14/14! +...
MAPLE
Q:= series(Int(sqrt(cosh(2*t)), t), t, 100):
S:= series(RootOf(Q-y, t), y, 100):
seq(coeff(S, y, j)*j!, j=1..100, 2); # Robert Israel, Aug 13 2017
MATHEMATICA
terms = 17; m = 2 terms; cc = CoefficientList[InverseSeries[Integrate[Sqrt[ Cosh[2 x]] + O[x]^m, x], x], x]; DeleteCases[cc * Range[0, m-1]!, 0] (* Jean-François Alcover, Apr 02 2019 *)
PROG
(PARI) {a(n) = my(L=x); for(i=1, n, L = intformal( 1/sqrt(cosh(2*L + O(x^(2*n+2)))) )); (2*n-1)!*polcoeff(L, 2*n-1)}
for(n=1, 20, print1(a(n), ", "))
(PARI) {a(n) = my(L=x); L = serreverse( intformal( sqrt(cosh(2*x + O(x^(2*n+2)))) )); (2*n-1)!*polcoeff(L, 2*n-1)}
for(n=1, 20, print1(a(n), ", "))
CROSSREFS
KEYWORD
sign
AUTHOR
Paul D. Hanna, Aug 13 2017
STATUS
approved