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Favard's theorem

In mathematics, Favard's theorem, also called the Shohat–Favard theorem, states that a sequence of polynomials satisfying a suitable three-term recurrence relation is a sequence of orthogonal polynomials. The theorem was introduced in the theory of orthogonal polynomials by {{harvs|txt|last=Favard|authorlink=Jean Favard|year=1935}} and {{harvtxt|Shohat|1938}}, though essentially the same theorem was used by Stieltjes in the theory of continued fractions many years before Favard's paper, and was rediscovered several times by other authors before Favard's work.

Statement

Suppose that y0 = 1, y1, ... is a sequence of polynomials where yn has degree n. If this is a sequence of orthogonal polynomials for some positive weight function then it satisfies a 3-term recurrence relation. Favard's theorem is roughly a converse of this, and states that if these polynomials satisfy a 3-term recurrence relation of the form

: y_{n+1}= (x-c_n)y_n - d_n y_{n-1}

for some numbers cn and dn,

then the polynomials yn form an orthogonal sequence for some linear functional Λ with Λ(1)=1; in other words Λ(ymyn) = 0 if m ≠ n.

The linear functional Λ is unique, and is given by Λ(1) = 1, Λ(yn) = 0 if n > 0.

The functional Λ satisfies Λ(y{{su|b=n|p=2}}) = dn Λ(y{{su|b=n–1|p=2}}), which implies that Λ is positive definite if (and only if) the numbers cn are real and the numbers dn are positive.

See also

References

  • {{Citation | title=An introduction to orthogonal polynomials | url=https://books.google.com/books?id=IkCJSQAACAAJ | year=1978 | last1=Chihara | first1=Theodore Seio | series=Mathematics and its Applications | volume=13| location=New York | publisher=Gordon and Breach Science Publishers | isbn=978-0-677-04150-6 | mr=0481884 }} Reprinted by Dover 2011, {{ISBN|978-0-486-47929-3}}
  • {{Citation | last1=Favard | first1=J. | title=Sur les polynomes de Tchebicheff. | url=https://gallica.bnf.fr/ark:/12148/bpt6k3152t/f2052.item | language=French | jfm=61.0288.01 | year=1935 | journal=C. R. Acad. Sci. Paris | volume=200 | pages=2052–2053}}
  • {{citation | last1=Rahman | first1=Q. I. | last2=Schmeisser | first2=G. | title=Analytic theory of polynomials | series=London Mathematical Society Monographs. New Series | volume=26 | location=Oxford | publisher=Oxford University Press | year=2002 | isbn=0-19-853493-0 | zbl=1072.30006 | pages=15–16 }}
  • {{eom|title=Favard theorem|id=f/f038300|first=Yu. N.|last= Subbotin}}
  • {{Citation | last1=Shohat | first1=J. | title=Sur les polynômes orthogonaux généralises. | language=French | zbl=0019.40503 | year=1938 | journal=C. R. Acad. Sci. Paris | volume=207 | pages=556–558}}

Category:Orthogonal polynomials

Category:Theorems in approximation theory