# Is inversion always cheap with Twisted Edwards curves?

I'm reading on Jubjub, which is planned for the next upgrade of Zcash. It is based on a Twisted Edwards curve with parameters $$a = -1$$ and $$d = −(10240/10241)$$. The reading says Jubjub does not need projective coordinates because inversion is cheap.

My question is, is the "cheap inversion" property that of Twisted Edwards curves in general, or the parameter selection of Jubjub in particular?

• Thanks @user679128. Does that mean "inversion is cheap" for Jubjub in particular? Or is it the case for all Twisted Edward curves with a = -1? Or is it dependent on a and d? Or maybe just d? Or maybe something else? – jww Jun 7 at 12:24