# linear programming problem pivot selection

I am using pythons linprog to solve the following linear program. I am using a callback to have linprog display the tableau on each iteration below. I don't understand how the last tableau is calculated. I would think the pivot would be 1 (row [0],col [1]) but it seems like linprog selects -2(row[1],col[1]). I thought in a linear program the pivot had to be greater than 0? And actually linprog prints out (nan,nan) for pivot but it seems as though the -2 is used to reduce the problem.

x[1] + x[2] <= 10
-x[1] + x[2] <= -10
max f = x[1] + 2*x[2]


 [[ 1. 1. 1. 0. 0. 10.] [ 1. -1. -0. -1. 1. 10.] [ -1. -2. 0. 0. 0. 0.] [ -1. 1. 0. 1. 0. -10.]] (0, 0)   [[ 1. 1. 1. 0. 0. 10.] [ 0. -2. -1. -1. 1. 0.] [ 0. -1. 1. 0. 0. 10.] [ 0. 2. 1. 1. 0. 0.]] (nan, nan) 

 [[ 1. 0. 0.5 -0.5 10. ] [ -0. 1. 0.5 0.5 -0. ] [ 0. 0. 1.5 0.5 10. ]] (nan, nan) 

• At the options you can choose if the Bland’s anti-cycling rule will be applied or not. This could be the problem. – callculus Nov 29 '16 at 19:26