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# problem.chgQObj

Purpose
Used to change a single quadratic coefficient in the objective function corresponding to the variable pair (objqcol1,objqcol2) of the Hessian matrix.
Topic areas
Problem Modification, Quadratic, Optimizer
Synopsis
`problem.chgQObj(objqcol1, objqcol2, objqcoef)`
Arguments

objqcol1: var | Column index for the first variable in the quadratic term.
objqcol2: var | Column index for the second variable in the quadratic term.
objqcoef: float | New value for the coefficient in the quadratic Hessian matrix. If an entry in objqcoef is 0, the corresponding entry will be deleted.

Further information
1. If objqcol1 is not equal to objqcol2, then both the matrix elements (objqcol1, objqcol2) and (objqcol2, objqcol1) are changed to leave the Hessian symmetric.
2. The quadratic matrix coefficients are implicitly divided by two. But since the coefficients for xixj ( i not equal to j) appear twice, only the coefficients for xi2 should be multiplied by two in the objqcoef argument to account for the implicit division. See the example above.
3. Arguments objqcol1, objqcol2 may contain either xpress.var objects or integer indices.
Related topics
problem.chgCoef, problem.chgMCoef, problem.chgMQObj, problem.chgObj, problem.getQObj.
Parent Topic Xpress problem object methods

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