Joachim König, and Dmitri Nedrenco, Septic equations are solvable by 2-fold
Forum Geometricorum, 16 (2016) 193--205.
Abstract. In this paper we prove that a generic polynomial equation of degree 7 over the rationals is solvable by 2-fold origami. In particular we show how to septisect an arbitrary angle and to take arbitrary seventh roots. This extends the work of Alperin, Lang (2006) and Nishimura~(2015) on 2-fold origami and significantly improves previously known results on the solvability of septic equations by multi-fold origami. Furthermore we give exact crease patterns for folding polynomials with Galois groups A_7 resp. PSL_3F_2.
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