Where to find it:

NodeGraph / Right Mouse Click / Add Nodes / Math / Extension Pack / A /

return the arccosine of the parameter

atan returns the angle whose trigonometric cosine is x. 

The range of values returned by acos is [0,π]. The result is undefined if |x|>1.

https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/acos.xhtml

•X

Specify the value whose arccosine to return.

•Output RGB

Separate computation for Input Channels Red, Green, Blue with an Alpha Channel of 1.0

•R

Computation for Input Red channel only, saved into R, G and B channel with an Alpha of 1.0

•G

Computation for Input Green channel only, saved into R, G and B channel with an Alpha of 1.0

•B

Computation for Input Blue channel only, saved into R, G and B channel with an Alpha of 1.0

•A

Computation for Input Alpha channel only, saved into R, G and B channel with an Alpha of 1.0