 Usage:  z = pdfb (x,a,b)   
 
 Input:

  x                      array 
                         
  a                      array or scalar 
                         
  b                      array or scalar 
                         
                         
                         
 Output:

  z                      array 
                         
                         
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(C) MD*TECH Method and Data Technologies, 17.8.2000
