# Multiplicative inverse of polynomial modulus an integer

How do you calculate the multiplicative inverse of a polynomial mod a monomial/integer?The specific questions are: Find the multiplicative inverse of 1) x+1 mod 3 2) x^2+x-1 mod 3 3) x^2+x-1 mod 32

I understand that you need to use the Extended Euclidean algorithm to solve it. For integer mod integer (e.g. 11 mod 26) and polynomial mod polynomial, its clear.But how to solve x+1 mod 3?

Neither $$x+1$$ nor $$x^2+x-1$$ are invertible modulo $$3$$, i.e. in the polynomial ring $$\mathbf Z/3\mathbf Z[x]$$, which is a polynomial ring over a field, because in such a case, for any polynomials $$f,g$$ we have $$\deg fg=\deg f+\deg g \ge\deg f,\deg g$$, which is not compatible with $$fg=1$$ unless $$f$$ and $$g$$ are constants.
If the quotient ring is not a field, this may not be true. However $$x^2+x-1$$ invertible in ,is impossible since its leading coefficient is $$1$$, hence $$\deg f(x)(x^2+x-1)=\deg f+2$$.
For an example with coefficients in $$\mathbf Z/32\mathbf Z$$ you can check that $$\;(16x^2-4x+1)(4x+1)=1$$.
• do you mean that for a polynomial f to have a multiplicative inverse mod g then inlinedeg fg=deg f+deg g ≥deg f,deg g ? If that is the case then x+1 mod 3 meets the criteria because deg ((x+1)3)=1' and deg f=1 and deg g=0` . Kindly explain – jaynjayn Jul 15 '15 at 7:03
• If $fg=1$, we must have $\deg f+\deg g=\deg 1=0$, whence $\deg f=\deg g=0$, i. e. $f$ and $g$ must be non-zero constants. – Bernard Jul 15 '15 at 7:29