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Bibliography

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G. Birkhoff, Generalized arithmetic, Duke Math. J. vol. 9 (1942) pp. 283-302. MR 0007031 (4:74g)

[2]

M. M. Day, Convergence, closure and neighborhoods, Duke Math. J. vol. 11 (1944) pp. 181-199. MR 0009844 (5:212b)

[3]

-, Oriented systems, Duke Math. J. vol. 11 (1944) pp. 201-229. MR 0009970 (5:231c)

[4]

A. N. Whitehead and B. Russell, Principia mathematica, Cambridge, University Press.