↑ Colebrooke, Henry-Thomas (1817), Algebra, with arithmetic and mensuration, from the Sanscrit of Brahmegupta and Bhascara, John Murray, p. 58. ↑ Ball, D.G. (1973), "A generalisation of π", Mathematical Gazette, 57 (402): 298–303, doi:10.2307/3616052, Griffiths, David; Culpin, David (1975), "Pi-optimal polygons", Mathematical Gazette, 59 (409): 165–175, doi:10.2307/3617699. 1 2 de Villiers, Michael (2009), Some Adventures in Euclidean Geometry, Dynamic Mathematics Learning, p. 58, ISBN 9780557102952.