Problems with the plot of a function.

Question

For some reason the result of Plot of the following function:

Clear[x]; Clear[a]; Clear[b];

m={{(1+1/k)/x^2,1/k},{-1/k,x^2(1-1/k)}};
{{l1,l2},{v1,v2}}=Eigensystem[m];
g = Solve[{0,1}==a v1 + b v2,{a,b}];
alpha = First[a /. g];
beta = First[b /. g];
d[x_, h_, n_]:= alpha l1^n v1 + beta l2^n v2 /. k-> (2 Log[x])/h
Plot[Last[d[x,4,8]], {x,6.78,8}, PlotStyle->Orange]

is this

enter image description here

How can I remove those blank spaces to see the plot of a continuous function?

Welcome to Mathematica.SE! I suggest that: 1) You take the introductory Tour now! 2) When you see good questions and answers, vote them up by clicking the gray triangles, because the credibility of the system is based on the reputation gained by users sharing their knowledge. Also, please remember to accept the answer, if any, that solves your problem, by clicking the checkmark sign! 3) As you receive help, try to give it too, by answering questions in your area of expertise. — bbgodfrey, Feb 19 '15 at 15:03
The plot displays a nice continuous curve in my machine. Try starting a fresh Mathematica session. ¡Suerte! — Dr. belisarius, Feb 19 '15 at 15:15
I started a new session and the problem remains the same, but now I know it is a particular problem of mine. ¡Gracias! — Antoñito, Feb 19 '15 at 15:25
The problem are imaginary residuals. You need to Chop your function, then everything is fine. — Yves Klett, Feb 19 '15 at 15:39
@Mr.Wizard it is a simple mistake, but one that might occur frequently (and I was stumped belisarius did not catch it immediately). Not sure about closure, but perhaps there is a suitable duplicate around somewhere? — Yves Klett, Feb 19 '15 at 18:16
@Yves Sorry, you're right. (I did vote for your answer, by the way.) There are many questions on the site that relate to attempting to Plot complex values; would you help me find a good one to mark as the original in this case? — Mr.Wizard, Feb 19 '15 at 18:30
@Mr.Wizard what about this one: http://mathematica.stackexchange.com/q/34365/131 — Yves Klett, Feb 19 '15 at 20:26
@YvesKlett Can you appear for a short chat? Your link at top (1: ) did not work. — Narasimham, Apr 21 '15 at 11:07

score 4 · Answer 1 · answered Feb 19 '15 at 15:41

Your function seems to produce values with small imaginary components, which Plot deliberately ignores. If you Chop your results, the function seems to plot just fine:

m = {{(1 + 1/k)/x^2, 1/k}, {-1/k, x^2 (1 - 1/k)}};
{{l1, l2}, {v1, v2}} = Eigensystem[m];
g = Solve[{0, 1} == a v1 + b v2, {a, b}];
alpha = First[a /. g];
beta = First[b /. g];
d[x_, h_, n_] := alpha l1^n v1 + beta l2^n v2 /. k -> (2 Log[x])/h
Plot[Last[d[x, 4, 8]] // Chop, {x, 6.78, 8}, PlotStyle -> Orange]

Mathematica graphics

Thank you very much. That's it! – Antoñito Feb 19 '15 at 15:42 — Antoñito, Feb 19 '15 at 15:42

Problems with the plot of a function.

1 Answers1

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