## Compute in a polynomial ring

Let $p(x) = 2x^3 - 3x^2 + 4x - 5$ and let $q(x) = 7x^3 + 33x - 4$. In each of parts (a), (b), and (c), compute $p+q$ and $pq$ under the assumption that the coefficients of the two given polynomials are taken from the specified ring (where the integer coefficients are taken mod $n$ in parts (b) and (c)): (a) $R = \mathbb{Z}$, (b) $R = \mathbb{Z}/(2)$, $R = \mathbb{Z}/(3)$.

1. Evidently, $p(x)+q(x) = 9x^3 - 3x^2 + 37x - 9$ and $p(x)q(x) = 14x^6 - 21x^5 + 94x^4$ $- 142 x^3 + 144 x^2 - 181 x + 20$.
2. In $\mathbb{Z}/(2)$, $p(x) = x^2 + 1$ and $q(x) = x^3 + x$. Evidently, $P(x) + q(x) = x^3 + x^2 + x + 1$ while $p(x)q(x) = x^5 + x$.
3. In $\mathbb{Z}/(3)$, $p(x) = 2x^3 + x + 1$ and $q(x) = x^3 + 2$. Evidently, $p(x) + q(x) = x$ while $p(x)q(x) = 2x^6 + x^4 + + 2x^3 2x + 2$.

• Danny  On November 26, 2011 at 12:37 am

can you please tell me why in part 3, the ciefficient of x^3 in q is 2? I thought it`s 1.

• nbloomf  On November 28, 2011 at 10:34 am

That was a mistake on my part. Thanks!