# scipy.special.i1#

scipy.special.i1(x, out=None) = <ufunc 'i1'>#

Modified Bessel function of order 1.

Defined as,

$I_1(x) = \frac{1}{2}x \sum_{k=0}^\infty \frac{(x^2/4)^k}{k! (k + 1)!} = -\imath J_1(\imath x),$

where $$J_1$$ is the Bessel function of the first kind of order 1.

Parameters:
xarray_like

Argument (float)

outndarray, optional

Optional output array for the function values

Returns:
Iscalar or ndarray

Value of the modified Bessel function of order 1 at x.

iv

Modified Bessel function of the first kind

i1e

Exponentially scaled modified Bessel function of order 1

Notes

The range is partitioned into the two intervals [0, 8] and (8, infinity). Chebyshev polynomial expansions are employed in each interval.

This function is a wrapper for the Cephes  routine i1.

References



Cephes Mathematical Functions Library, http://www.netlib.org/cephes/

Examples

Calculate the function at one point:

>>> from scipy.special import i1
>>> i1(1.)
0.5651591039924851


Calculate the function at several points:

>>> import numpy as np
>>> i1(np.array([-2., 0., 6.]))
array([-1.59063685,  0.        , 61.34193678])


Plot the function between -10 and 10.

>>> import matplotlib.pyplot as plt
>>> fig, ax = plt.subplots()
>>> x = np.linspace(-10., 10., 1000)
>>> y = i1(x)
>>> ax.plot(x, y)
>>> plt.show()