Returns the arcsine of its argument, in radians.
Implemented by: zx81, spectrum, BBC, bw32
With variations:
ASN is also found, with this name, in historical implementations of BASIC for IBM System/360 and System/370, according to the documentation of Bywater BASIC 3.20.
The BASIC Cookbook points out that BASICs which don't implement the function can achieve the same results, for a given X argument, with the following formula:
ATN(X/SQR(X*X+1))