Function j1f_asympt_alpha 

pub(crate) fn j1f_asympt_alpha(x: f64) -> f64

Note expansion generation below: this is negative series expressed in Sage as positive, so before any real evaluation x=1/x should be applied.

Generated by SageMath:

def binomial_like(n, m):
    prod = QQ(1)
    z = QQ(4)*(n**2)
    for k in range(1,m + 1):
        prod *= (z - (2*k - 1)**2)
    return prod / (QQ(2)**(2*m) * (ZZ(m).factorial()))

R = LaurentSeriesRing(RealField(300), 'x',default_prec=300)
x = R.gen()

def Pn_asymptotic(n, y, terms=10):
    # now y = 1/x
    return sum( (-1)**m * binomial_like(n, 2*m) / (QQ(2)**(2*m)) * y**(QQ(2)*m) for m in range(terms) )

def Qn_asymptotic(n, y, terms=10):
    return sum( (-1)**m * binomial_like(n, 2*m + 1) / (QQ(2)**(2*m + 1)) * y**(QQ(2)*m + 1) for m in range(terms) )

P = Pn_asymptotic(1, x, 50)
Q = Qn_asymptotic(1, x, 50)

R_series = (-Q/P)

# alpha is atan(R_series) so we're doing Taylor series atan expansion on R_series

arctan_series_Z = sum([QQ(-1)**k * x**(QQ(2)*k+1) / RealField(700)(RealField(700)(2)*k+1) for k in range(25)])
alpha_series = arctan_series_Z(R_series)

# see the series
print(alpha_series)

See notes/bessel_asympt.ipynb for generation