The Number Concept: Its Origin and Development. E-book. Formato PDF Levi Leonard Conant   -  Forgotten Books, 2017  - 

Sincere thanks are due to those who have assisted the author in the preparation of his materials. Especial acknowledgment should be made to Horatio Hale, Dr. D. G. Brinton, Frank Hamilton Cushing, and Dr. A. F. Chamberlain.