Morin surface

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The Morin surface has a quadruple point through which passes its axis of symmetry.

The diagram shows section East bounded by three loops: ABCDA, AEFGA, and AHIJA.

The third loop, AHIJA, is a line of double points where section East intersects with itself.

Morin surface can be elegantly described by a set of equations[1] in either open version (with poles sent to infinity) or closed.