The $6π$ theorem in minimal surfaces. Beeson, M. Pacific Journal of Mathematics, 117:17-25, 1985.  The second link gives comments (written 2006) that simplify the proof and answer a question raised by Nitsche.
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Pdf2  bibtex   @article{beeson1985b,
	author = {Michael Beeson},
	date-added = {2014-11-14 16:50:41 +0000},
	date-modified = {2014-11-17 18:13:34 +0000},
	journal = {Pacific Journal of Mathematics},
	keywords = {Minimal surfaces},
	note = {The second link gives comments (written 2006) that simplify the proof and answer a question raised by Nitsche.},
	pages = {17-25},
	title = {The $6\pi$ theorem in minimal surfaces},
	url_pdf = {sixpi.pdf},
	url_pdf2 = {sixpiReview.pdf},
	volume = {117},
	year = {1985}} 
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