[texhax] crossreference in bibtex

[Zbigniew Nitecki]

I'm confused by the behavior of bibtex with respect to a  
crossreference entry.  This behavior seems to have started recently  
(ie, it was accepting things normally before).
I've attached my .bib file
and here is the calculus console response to a typeset bibtex  
command (I'm using TeXShop):

This is BibTeX, Version 0.99c (Web2C 7.5.4)
The top-level auxiliary file: Calculus.aux
The style file: plain.bst
Database file #1: calculus.bib
A bad cross reference---entry "Newton:Principia"
refers to entry "Cohen", which doesn't exist
Warning--I didn't find a database entry for "Cohen"
Warning--empty publisher in Newton:Principia
Warning--empty year in Newton:Principia
(There was 1 error message)

What is going on?  I've checked to see if there are any parenthesis  
errors in the .bib  file, and I find none.


Zbigniew Nitecki
Department of Mathematics
Tufts University
Medford, MA 02155

telephones:
Office    (617)627-3843
Dept.    (617)627-3234
Dept. fax    (617)627-3966

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