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## Re: [Bug-glpk] 0-ary slice not allowed

**From**: |
Andrew Makhorin |

**Subject**: |
Re: [Bug-glpk] 0-ary slice not allowed |

**Date**: |
Thu, 27 Nov 2008 04:28:16 +0300 |

>*>> Just for clarity.*
>*>> Do you agree that the following notation is meaningless?*
>*>> sum{(2,3) in E} c[i,j] * x[i,j] *
>* The following model is correctly solved by GLPK:*
>* set E := {(2,3)};*
>* var v{(i,j) in E};*
>* s.t. con1 {(i,j) in E} : v[i,j] - sum{(2,k) in E : k == j} 1 = 0;*
>* var w{(i,j) in E};*
>* s.t. con2 {(i,j) in E} : w[i,j] - sum{(k,3) in E : k == i} 1 = 0;*
>* solve;*
>* display v,w;*
>* Why shoudn't*
>* s.t. con1 {(i,j) in E} : v[i,j] - sum{(2,3) in E} 1 = 0;*
>* work?*
I do not know what to say. The notation sum{(2,3) in E} is just
mathematically incorrect. It can be written in other, correct ways,
say, sum{(i,j) in E: i = 2 and j = 3} or (if (2,3) in E then ...), so
in this case it is reasonable to follow the principle "Entia non sunt
multiplicanda praeter necessitatem" :)