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Depends if you want to input the series and get the sum of the series (in that case follow Eric's answer), or the converse, in which case you can ask for the series expansion.

sage: f = 1/(1-x)
sage: f.series(x)
1 + 1*x + 1*x^2 + 1*x^3 + 1*x^4 + 1*x^5 + 1*x^6 + 1*x^7 + 1*x^8
+ 1*x^9 + 1*x^10 + 1*x^11 + 1*x^12 + 1*x^13 + 1*x^14 + 1*x^15
+ 1*x^16 + 1*x^17 + 1*x^18 + 1*x^19 + Order(x^20)