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Cash and cash equivalents and other investments (Tables)
12 Months Ended
Dec. 31, 2018
Cash and cash equivalents [abstract]  
Schedule of Cash and cash equivalents

Cash and cash equivalents are as follows:

 

  December 31,
  2 0 1 8   2 0 1 7
           
Cash $ 514,166   $ 1,339,887
Cash equivalents (1)   6,473,075     5,863,717
  $ 6,987,241   $ 7,203,604

 

(1)Most of these cash equivalents correspond to Treasury Bills issued by the United States Treasury Department, whose initial original maturity is less than 90 days.