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H.E. Chrestenson (1964) American Mathematical Monthly 71(7): 813
H.J. Ricardo (1975) American Mathematical Monthly 82(6): 681–4
G.M. Kaufman (1963) American Mathematical Monthly 70(10): 1116
G. C. Dorner (1971) Mathematics Magazine 44(4): 223–6
G. C. Dorner (1971) Mathematics Magazine 44(4): 223–6
G. C. Dorner (1971) Mathematics Magazine 44(4): 223–6
G. C. Dorner (1971) Mathematics Magazine 44(4): 223–6
J.D. Emerson & K. Larson (1981) American Mathematical Monthly 88(5): 357