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EXAMPLE
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1 - x - 2*x^2 - 10*x^3 - 74*x^4 - 706*x^5 - 8162*x^6 - 110410*x^7 - ...
The coefficients in A(x)^n begin:
n=1: [1, -1, -2, -10, -74, -706, -8162, -110410, ...];
n=2: [1, -2,(-3),-16, -124, -1224, -14516, -200192, ...];
n=3: [1, -3,(-3),-19, -156, -1596, -19412, -272772, ...];
n=4: [1, -4, -2,(-20),-175, -1856, -23136, -331008, ...];
n=5: [1, -5, 0,(-20),-185, -2031, -25920, -377280, ...];
n=6: [1, -6, 3, -20,(-189),-2142, -27951, -413568, ...];
n=7: [1, -7, 7, -21,(-189),-2205, -29379, -441519, ...];
n=8: [1, -8, 12, -24, -186,(-2232),-30324, -462504, ...];
n=9: [1, -9, 18, -30, -180,(-2232),-30882, -477666, ...];
n=10:[1, -10, 25, -40, -170, -2212,(-31130),-487960, ...];
n=11:[1, -11, 33, -55, -154, -2178,(-31130),-494186, ...]; ...
where the coefficients in parenthesis demonstrate the properties:
(2) [x^n] A(x)^(2*n-2) = [x^n] A(x)^(2*n-1) for n>=2,
(3) [x^n] A(x)^(2*n-1) = -(2*n-1)*A000699(n) for n>=1:
A000699 = [1/1, 3/3, 20/5, 189/7, 2232/9, 31130/11, ...].
Note: g.f. of A000699, G(x), satisfies: G(x) = x + x^2*[d/dx G(x)^2/x].
(End)
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PROG
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(PARI) {a(n) = if( n<0, 0, polcoeff( 1 / sum( k=0, n, x^k * (2*k)! / (2^k * k!), x * O(x^n)), n))}
(PARI) {a(n)=local(A=1+x+x*O(x^n)); for(i=0, n, A=1-x*A^2*deriv(x/A^2)); polcoeff(A, n)} /* Paul D. Hanna, Mar 03 2012 */
(Sage)
if len == 1: return [1]
T = [0]*(2*len-1); T[1] = 1; R = [1, -1]
for n in (1..2*len-3):
a, b, c = 1, 0, 0
for k in range(n, -1, -1):
r = a-(k+2)*c
if k < n : T[k+2] = u;
a, b, c = T[k-1], a, b
u = r
T[1] = u;
if is_even(n): R.append(-abs(u))
return R
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