# zeroFilter -- returns solutions with k-th coordinate less than the given tolerance

## Synopsis

• Usage:
zeroSols = zeroFilter(sols,k,tol)
• Inputs:
• sols, a list, containing the solutions of a polynomial system
• k, an integer, index of a coordinate of a solution
• tol, , tolerance on the absolute value of the k-th coordinate
• Outputs:
• zeroSols, a list, solutions with k-th coordinate less than the given tolerance

## Description

A solution has its k-th coordinate zero when the abs function evaluates to a number less than or equal to the given tolerance.

 i1 : R = CC[x,y]; i2 : f = { x^3*y^5 + y^2 + x^2*y, x*y + x^2 - 1}; i3 : fSols = solveSystem(f, randomSeed=>3); i4 : fSols/print {-1, 0} {1, 0} {-1.59272, .964857} {.742585+.425943*ii, .270685-1.00715*ii} {-.764107, -.544612} {1.33076-.335184*ii, -.62414+.513163*ii} {-.894935-.624334*ii, .143333+1.14868*ii} {1.33076+.335184*ii, -.62414-.513163*ii} {-.894935+.624334*ii, .143333-1.14868*ii} {.742585-.425943*ii, .270685+1.00715*ii} o4 = {, , , , , , , , , } o4 : List

There is one solution with zero second coordinate:

 i5 : zeroSols = zeroFilter(fSols,1,1.0e-10); i6 : zeroSols / print {-1, 0} {1, 0} o6 = {, } o6 : List

Here is another system where we filter solutions with small' first coordinate:

 i7 : f = {x^2+y^2,y*x+x}; i8 : fSols = solveSystem(f); i9 : fSols/print (-2.46519e-32+1.07852e-32*ii, -2.40467e-32+9.0524e-34*ii) {-ii, -1} {ii, -1} o9 = {, , } o9 : List i10 : zeroSols = zeroFilter(fSols,0,1.0e-10); i11 : zeroSols/print (-2.46519e-32+1.07852e-32*ii, -2.40467e-32+9.0524e-34*ii) o11 = {} o11 : List`

Good values for the tolerance are relative to the accuracy and the condition number of the solution. To improve the accuracy of a solution, apply refineSolutions with a higher working precision.