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Pochhammer contour
In mathematics, the Pochhammer contour, introduced by and , is a contour in the complex plane with two points removed, used for contour integration. If ''A'' and ''B'' are loops around the two points, both starting at some fixed point ''P'', then the Pochhammer contour is the commutator ''ABA''−1''B''−1, where the superscript −1 denotes a path taken in the opposite direction. With the two points taken as 0 and 1, the fixed basepoint ''P'' being on the real axis between them, an example is the path that starts at ''P'', encircles the point 1 in the counter-clockwise direction and returns to ''P'', then encircles 0 counter-clockwise and returns to ''P'', after that circling 1 and then 0 clockwise, before coming back to ''P''. The class of the contour is an actual commutator when it is considered in the fundamental group with basepoint ''P'' of the complement in the complex plane (or Riemann sphere) of the two points looped. When it comes to taking contour integrals, moving basepoint from ''P'' to another choice ''Q'' makes no difference to the result, since there will be cancellation of integrals from ''P'' to ''Q'' and back.
== Homologous to zero but not homotopic to zero ==
Within the doubly punctured plane this curve is homologous to zero but not homotopic to zero. Its winding number about any point is 0 despite the fact that within the doubly punctured plane it cannot be shrunk to a single point.
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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