FUNCTION :  qseries[theta] - theta function                

CALLING SEQUENCE :  theta(z,q,T)       
    
PARAMETERS :   z   - variable              
               q   - variable 
               T   - positive integer 
   
SYNOPSIS :   

 theta(z,q,T) returns the tuncated theta-series                    

                                   T
                                 -----        2
                                  \      i  (i )
                                   )    z  q
                                  /
                                 -----
                                i = - T

EXAMPLES :   
   
-------------------------------------------------------------------------
> theta(-1,q,10);                           

    100      81      64      49      36      25      16      9      4
 2 q    - 2 q   + 2 q   - 2 q   + 2 q   - 2 q   + 2 q   - 2 q  + 2 q  - 2 q + 1

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SEE  ALSO :   jacprod, tripleprod, theta2, theta3, theta4