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2018-04-04 15:56:32 +0200 asked a question How do I plot a real function whose computation involves complex intermediate results?

I have an algebraic expressing ev for an eigenvalue of a 4x4 matrix and I want to plot it. The expression has square roots and cube roots, and sometimes there are complex intermediate results although the final result is real. With float, the imaginary part will be non-zero due to numeric errors, but I can just ignore it. Still, when I try to plot this function, I get error messages.

var("u")
ev = (1/2*u + 1/2*sqrt(2/3*u^2 - 1/9*(u^4 - 18*u)/(1/27*u^6 - u^3 +
1/6*sqrt(-32/3*u^6 + 252*u^3 + 9) + 1/2)^(1/3) - (1/27*u^6 - u^3 +
1/6*sqrt(-32/3*u^6 + 252*u^3 + 9) + 1/2)^(1/3) + 6/sqrt((u^4 +
3*(1/27*u^6 - u^3 + 1/6*sqrt(-32/3*u^6 + 252*u^3 + 9) + 1/2)^(1/3)*u^2 -
18*u + 9*(1/27*u^6 - u^3 + 1/6*sqrt(-32/3*u^6 + 252*u^3 + 9) +
1/2)^(2/3))/(1/27*u^6 - u^3 + 1/6*sqrt(-32/3*u^6 + 252*u^3 + 9) +
1/2)^(1/3))) + 1/6*sqrt((u^4 + 3*(1/27*u^6 - u^3 + 1/6*sqrt(-32/3*u^6 +
252*u^3 + 9) + 1/2)^(1/3)*u^2 - 18*u + 9*(1/27*u^6 - u^3 +
1/6*sqrt(-32/3*u^6 + 252*u^3 + 9) + 1/2)^(2/3))/(1/27*u^6 - u^3 +
1/6*sqrt(-32/3*u^6 + 252*u^3 + 9) + 1/2)^(1/3)))

print ev.subs(u=2.6)
print ev.subs(u=3.0)
print ev.subs(u=3.0).real()
plot(ev.subs(u=x).real(),(x,2.6,3))

Plotting will draw only from 2.6 to 2.62 and then abort with the following error message

verbose 0 (3749: plot.py, generate_plot_points) WARNING: When plotting, failed to evaluate function at 189 points. 
verbose 0 (3749: plot.py, generate_plot_points) Last error message: 'math domain error'

I tried this in the sage cell server. (On my outdated office computer installation, sage 7.4, I get a different error "AssertionError".)