Where to find it:

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

inversesqrt returns the inverse of the square root of x; 

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

•X

Specify the value of which to take the inverse of the square root.

•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