Math::Series defines a class for simple mathematic series with a recursive 
definition such as x_(n+1) = 1 / (x_n + 1). Such a recursive definition is 
treated as a sequence whose elements will be added to form a series. You 
can refer to the previous sequence element as well as to the current index 
in the series. Creation of a Math::Series object is described below in the 
paragraph about the constructor.

Math::Series uses Math::Symbolic to parse and modify the recursive 
sequence definitions. That means you specify the sequence as a string 
which is parsed by Math::Symbolic. Alternatively, you can pass the 
constructor a Math::Symbolic tree directly.

Because Math::Series uses Math::Symbolic for its implementation, all 
results will be Math::Symbolic objects which may contain other variables 
than the sequence variable and the iterator variable.

Each Math::Series object is an iterator to iterate over the elements of 
the series starting at the first element (which was specified by the 
starting element, the second argument to the new() constructor). It offers 
facilities to cache all calculated elements and access any element 
directly, though unless the element has been cached in a previous 
calculation, this is just a shortcut for repeated use of the iterator.

Every element in the series may only access its predecessor, not the 
elements before that.

WWW: http://search.cpan.org/dist/Math-Series/