In mathematics, a polynomial transformation consists of computing the polynomial whose roots are a given function of the roots of a polynomial. Polynomial transformations such as Tschirnhaus transformations are often used to simplify the solution of algebraic equations.
Simple examples
Translating the roots Let
P ( x ) = a 0 x n + a 1 x n − 1 + ⋯ + a n {\displaystyle P(x)=a_{0}x^{n}+a_{1}x^{n-1}+\cdots +a_{n}}
be a polynomial, and
α 1 , … , α n {\displaystyle \alpha _{1},\ldots ,\alpha _{n}}
be its complex roots (not necessarily distinct). For any constant c, the polynomial whose roots are
α 1 + c , … , α n + c {\displaystyle \alpha _{1}+c,\ldots ,\alpha _{n}+c}
is
Q ( y ) = P ( y − c ) = a 0 ( y − c ) n + a 1 ( y − c ) n − 1 + ⋯ + a n . {\displaystyle Q(y)=P(y-c)=a_{0}(y-c)^{n}+a_{1}(y-c)^{n-1}+\cdots +a_{n}.}
If the coefficients of P are integers and the constant c = p q {\displaystyle c={\frac {p}{q}}} is a rational number, the coefficients of Q may be not integers, but the polynomial cn Q has integer coefficients and has the same roots as Q. A special case is when c = a 1 n a 0 . {\displaystyle c={\frac {a_{1}}{na_{0}}}.} The resulting polynomial Q does not have any term in yn − 1.
Reciprocals of the roots Let
P ( x ) = a 0 x n + a 1 x n − 1 + ⋯ + a n {\displaystyle P(x)=a_{0}x^{n}+a_{1}x^{n-1}+\cdots +a_{n}}
be a polynomial. The polynomial whose roots are the reciprocals of the roots of P is its reciprocal polynomial
Q ( y ) = y n P ( 1 y ) = a n y n + a n − 1 y n − 1 + ⋯ + a 0 . {\displaystyle Q(y)=y^{n}P\left({\frac {1}{y}}\right)=a_{n}y^{n}+a_{n-1}y^{n-1}+\cdots +a_{0}.}
Scaling the roots Let
P ( x ) = a 0 x n + a 1 x n − 1 + ⋯ + a n {\displaystyle P(x)=a_{0}x^{n}+a_{1}x^{n-1}+\cdots +a_{n}}
be a polynomial, and c be a non-zero constant. A polynomial whose roots are the product by c of the roots of P is
Q ( y ) = c n P ( y c ) = a 0 y n + a 1 c y n − 1 + ⋯ + a n c n . {\displaystyle Q(y)=c^{n}P\left({\frac {y}{c}}\right)=a_{0}y^{n}+a_{1}cy^{n-1}+\cdots +a_{n}c^{n}.}
… excerpt ends here. Continue reading the full article.
