Question

Let \(g, h \in R[X, Y]\) with \(g=\sum_{i=0}^{m-1} g_{i} Y^{i}\) and \(h=\sum_{i=0}^{m-1} h_{i} Y^{i},\) where each \(g_{i}\) and \(h_{i}\) is a polynomial in \(X\) of degree less than \(k .\) The product \(f:=g h \in R[X, Y]\) may be written \(f=\sum_{i=0}^{2 m-2} f_{i} Y^{i},\) where each \(f_{i}\) is a polynomial in \(X .\) Show how to compute \(f,\) given \(g\) and \(h,\) using \(O(M(k m))\) operations in \(R\). Hint: for an appropriately chosen integer \(t>0,\) first convert \(g, h\) to \(\tilde{g}, \tilde{h} \in R[X],\) where \(\tilde{g}:=\sum_{i=0}^{m-1} g_{i} X^{t i}\) and \(\tilde{h}:=\sum_{i=0}^{m-1} h_{i} X^{t i} ;\) next, compute \(\tilde{f}:=\tilde{g} \tilde{h} \in R[X] ;\) finally, "read off" the \(f_{i}\) 's from the coefficients of \(\tilde{f}\).

Short answer

a) Step 1 b) Step 2 c) Step 3 d) Complexity Analysis Answer: a) Step 1

Step-by-step solution

Transforming g and h into tilde-g and tilde-h

Given polynomials \(g(X, Y)\) and \(h(X, Y)\), you need to transform them into \(\tilde{g}(X)\) and \(\tilde{h}(X)\) using a suitable positive integer \(t\). The transformation is defined as follows: \(\tilde{g}(X) = \sum_{i=0}^{m-1} g_{i} X^{ti}\) and \(\tilde{h}(X) = \sum_{i=0}^{m-1} h_{i} X^{ti}\). Choose \(t = k + 1\). This will ensure that the degree of each term in both \(\tilde{g}\) and \(\tilde{h}\) is unique as no two terms will have the same degree.

Compute the product tilde-f = tilde-g * tilde-h

Now that you have the transformed polynomials \(\tilde{g}(X)\) and \(\tilde{h}(X)\), you can compute the product \(\tilde{f}(X) = \tilde{g}(X) * \tilde{h}(X)\). This product can be computed using the standard polynomial multiplication algorithm or any other efficient multiplication algorithm, such as Karatsuba or FFT-based solvers.

Read coefficients of f from tilde-f

As a final step, you need to extract the coefficients of the polynomial \(f(X, Y)\) from the product \(\tilde{f}(X)\). For this, you can write \(\tilde{f}(X)\) in the form \begin{align*} \tilde{f}(X)=\sum_{i=0}^{m-1} (\sum_{j=0}^{m-1} f_{j}X^{(k+1)j}) X^{(k+1)i} = \sum_{i=0}^{m-1} \sum_{j=0}^{m-1} f_{i+j} X^{(k+1)(i+j)}. \end{align*} Now you can read off the coefficients of \(f(X, Y)\) by comparing the terms of \(\tilde{f}(X)\) with the terms of \(f(X, Y)\), which is given in the form: \(f(X, Y) = \sum_{i=0}^{2m-2} f_{i} Y^i\).

Complexity analysis

In each of the steps above, you perform at most \(O(km)\) coefficients operations. In the first step, you perform at most \(O(m)\) multiplications for each of the \(g_i\) and \(h_i\) terms. In the second step, the cost of multiplying two polynomials of degree \(O(km)\) is \(O(M(km))\) operations. In the third step, you perform \(O(km)\) operations to extract the \(f_i\) coefficients. Thus, the overall cost of computing the polynomial product using this method is \(O(M(km))\) operations in \(R\).

Key concepts

Polynomial Transformation

When working with polynomials like \(g(X, Y)\) and \(h(X, Y)\), it's often useful to transform them into a different form to simplify operations such as multiplication. This exercise involves transforming these bivariate polynomials into univariate polynomials \(\tilde{g}(X)\) and \(\tilde{h}(X)\).
This process, known as polynomial transformation, involves substituting each term of the polynomial with a power of \(X\). The transformation is defined by:\[ \tilde{g}(X) = \sum_{i=0}^{m-1} g_{i} X^{ti} \text{ and } \tilde{h}(X) = \sum_{i=0}^{m-1} h_{i} X^{ti} \]
Here, each \(g_i\) and \(h_i\) are polynomials in \(X\), and \(t\) is chosen such that all terms have unique powers. In our case, \(t\) is taken as \(k + 1\), ensuring that the powers of \(X\) do not overlap. This transformation effectively simplifies the multiplication process by allowing us to use efficient univariate multiplication algorithms.

Polynomial Coefficients

After transforming and multiplying the polynomials, the next step is to extract the coefficients of the resulting polynomial, \(\tilde{f}(X)\). This polynomial, \(\tilde{f}(X)\), is a product of two transformed polynomials \(\tilde{g}(X)\) and \(\tilde{h}(X)\).
It's important to "read off" or interpret these coefficients correctly to express the product in its original two-variable form, \(f(X, Y)\). The result of the polynomial product \(\tilde{f}(X)\) can be written as\[ \tilde{f}(X)=\sum_{i=0}^{m-1} \left(\sum_{j=0}^{m-1} f_{j}X^{(k+1)j}\right) X^{(k+1)i} = \sum_{i=0}^{m-1} \sum_{j=0}^{m-1} f_{i+j} X^{(k+1)(i+j)} \]
By analyzing the resulting powers and coefficients of \(\tilde{f}(X)\), it's possible to determine the coefficients \(f_i\) associated with \(Y^i\) in the original polynomial representation \(f(X, Y)\). This requires careful comparison of terms to ensure accurate reconstruction of the polynomial in its initial form.

Complexity Analysis

Understanding the complexity of algorithms is crucial for assessing their efficiency. In the context of polynomial multiplication, complexity analysis focuses on the number of operations needed to achieve specific transformations and products.
The transformed polynomials \(\tilde{g}(X)\) and \(\tilde{h}(X)\) require \(O(m)\) multiplications for constructing terms from \(g_i\) and \(h_i\). After transforming, the most computationally expensive step is the multiplication of these transformed polynomials. Using efficient polynomial multiplication algorithms, like Karatsuba or Fast Fourier Transform (FFT) methods, this can be performed in \(O(M(km))\) operations. Here, \(M(km)\) represents the number of operations required to multiply two polynomials of degree \(km\).
Finally, extracting coefficients from \(\tilde{f}(X)\) involves \(O(km)\) operations, which are relatively easy compared to the multiplication itself. Thus, the overall complexity of this polynomial multiplication approach is dominated by the multiplication step, making it \(O(M(km))\). This efficient complexity ensures that the method is optimal for handling large degree polynomials.