Question about evaluation with Sum

*To*: mathgroup at smc.vnet.net*Subject*: [mg19587] Question about evaluation with Sum*From*: CORNIL Jack Michel <jmcornil at club-internet.fr>*Date*: Sat, 4 Sep 1999 01:34:25 -0400*Sender*: owner-wri-mathgroup at wolfram.com

Hello, I have a big problem of evaluation with the function Sum. With the very simple following example, p=k Sum[p,{k, 1, n}] p k Sum[Evaluate[p], {k, 1, n}] 1 - n (1+n) 2 Sum[p,{k, 1, 5}] 15 how can I explain my students that in the first case p is left unevaluate and it needs Evaluate, in the last case p is evaluate correctly ? I do not understand how the "Attributes" of Sum can explain this. Thank You in advance Jack Michel CORNIL Versailles FRANCE