B101 Arc Tangent Function

Function subprogram ATG calculates, for real arguments $x$₁ and $x$₂ , $(x$₁,x₂) ≠(0.,0.) , an angle $\alpha$ such that

$\alpha =arctan(x$₁/x₂)&quad;and&quad;0≤α<2π.

Note that using the Fortran intrinsic function ATAN2 instead of ATG would result in

Structure:

FUNCTION subprogram

User Entry Names: ATG

Usage:

In any arithmetic expression, ATG(X1,X2)

has the value of $\alpha$ (in radians). ATG, X1 and X2 are of type REAL.

Notes:

This function subprogram is equivalent to the statement function ATG(X1,X2)=ATAN2(X1,X2)+(PI-SIGN(PI,X1))

where $PI\; =\; \pi$ .
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B102