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CAS is designed to calculate things symbolically, so sometimes we can be surprised with results for numerical calculations. e.g. SUM(1/x^2,x,1,100) tries to sum the expression (1/x^2) for x = 1 to 100. However, under CAS, it will result in a ratio of 2 very large integers. You'll have to run approx() to get the decimal result. If you try it with 5000 instead of 100, it will take quite a while and when complete will result in undef as it ran out of decimal digits for the numerator and denominator of the result. It's not trying to evaluate 1/x^2 in a loop and adding the result to a sum. It's trying to evaluate it in a way that it could do it with SUM(1/y^2,y,x,1000x) resulting in Psi(x,1)-Psi(1000*x+1,1). To use a conventional numerical loop, don't use CAS. In this case, go to home mode and use the Σ symbol from the math template key….as in Σ(1/x^2,x,1,5000). This runs much faster than SUM() as it's not trying to solve symbolically, and will result in a numerical solution (1.6447… in this case). Just remember the rule that CAS is trying to solve symbolically, Home solves numerically.