Math 2.0

Commutative algebra Definition

Polynomial evaluation

Builds on Polynomial ring

Definition

Let RR be a ring, let f=a0+a1x+⋯+anxn∈R[x]f = a_0 + a_1 x + \dots + a_n x^n \in R[x] and let c∈Rc \in R. The value of ff at cc is the element f(c)=a0+a1c+a2c2+⋯+ancn∈R.f(c) = a_0 + a_1 c + a_2 c^2 + \dots + a_n c^n \in R.

Remarks

The value f(c)f(c) does not depend on the choice of nn, because the extra coefficients are 00. For f=x2−2f = x^2 - 2 in ℤ[x]\mathbb{Z}[x], f(3)=7f(3) = 7. If RR is commutative, then (fg)(c)=f(c)g(c)(f g)(c) = f(c) g(c) and (f+g)(c)=f(c)+g(c)(f + g)(c) = f(c) + g(c). This gives the evaluation homomorphism. Horner's rule computes f(c)=a0+c(a1+c(a2+⋯+can))f(c) = a_0 + c(a_1 + c(a_2 + \dots + c a_n)) with nn multiplications.

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Further reading

  1. Web https://en.wikipedia.org/wiki/Polynomial_ring, Polynomial evaluation
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