$\sum_{i=1}^nx_i$
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\[\usepackage[all]{xy}\xymatrix@C+2em@R+2em{Q \ar@/_10pt/[ddr]_{q_1} \ar@/^10pt/[drr]^{q_2} \ar@{-->}[dr]|*+<3pt,3pt>{\scriptstyle u} & & \\& P \ar[d]_(0.4){p_1} \ar[r]^(0.4){p_2} & Y \ar[d]^{g}\\& X \ar[r]_{f} & Z}\]
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