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Bendixson–Dulac theorem
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Bendixson–Dulac theorem : ウィキペディア英語版
Bendixson–Dulac theorem
In mathematics, the Bendixson–Dulac theorem on dynamical systems states that if there exists a C^1 function \varphi(x, y) (called the Dulac function) such that the expression
:\frac + \frac
has the same sign (\neq 0) almost everywhere in a simply connected region of the plane, then the plane autonomous system
: \frac = f(x,y),
: \frac = g(x,y)
has no periodic solutions lying entirely within the region. "Almost everywhere" means everywhere except possibly in a set of measure 0, such as a point or line.
The theorem was first established by Swedish mathematician Ivar Bendixson in 1901 and further refined by French mathematician Henri Dulac in 1933 using Green's theorem.
==Proof==
Without loss of generality, let there exist a function \varphi(x, y) such that
:\frac +\frac >0
in simply connected region R. Let C be a closed trajectory of the plane autonomous system in R. Let D be the interior of C. Then by Green's Theorem,
:\iint _^ +\frac \right) dxdy } =\oint _^
:=\oint _^ dy \right) }.
But on C, dx=\dot dt and dy=\dot dt, so the integral evaluates to 0. This is a contradiction, so there can be no such closed trajectory C.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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