asin(x: array, /) array

Calculates an implementation-dependent approximation of the principal value of the inverse sine for each element x_i of the input array x.

Each element-wise result is expressed in radians.


The principal value of the arc sine of a complex number \(z\) is

\[\operatorname{asin}(z) = -j\ \ln(zj + \sqrt{1-z^2})\]

For any \(z\),

\[\operatorname{asin}(z) = \operatorname{acos}(-z) - \frac{\pi}{2}\]


For complex floating-point operands, asin(conj(x)) must equal conj(asin(x)).


The inverse sine (or arc sine) is a multi-valued function and requires a branch cut on the complex plane. By convention, a branch cut is placed at the line segments \((-\infty, -1)\) and \((1, \infty)\) of the real axis.

Accordingly, for complex arguments, the function returns the inverse sine in the range of a strip unbounded along the imaginary axis and in the interval \([-\pi/2, +\pi/2]\) along the real axis.

Note: branch cuts follow C99 and have provisional status (see Branch Cuts).


x (array) – input array. Should have a floating-point data type.


out (array) – an array containing the inverse sine of each element in x. The returned array must have a floating-point data type determined by Type Promotion Rules.


Special cases

For real-valued floating-point operands,

  • If x_i is NaN, the result is NaN.

  • If x_i is greater than 1, the result is NaN.

  • If x_i is less than -1, the result is NaN.

  • If x_i is +0, the result is +0.

  • If x_i is -0, the result is -0.

For complex floating-point operands, special cases must be handled as if the operation is implemented as -1j * asinh(x*1j).

Changed in version 2022.12: Added complex data type support.