# FindMinimum for a large range

I have a function functionSL[0.01,zl,zh] where I want to find at which zl its minimum value occurs for each different zh. I already know at which zl is the minimum for each zh since I calculated it using FindMinimum for each one. The problem now is that I want to use Table to calculate them all at the same time.

My code is as follows

d = 3;
toroot[a_?NumericQ, t_?NumericQ, zl_?NumericQ, zh_?NumericQ] := a - ((2 zl Sqrt[(1 + t^2 (1 - (zl/zh)^(d + 1))^-1)^-1])/((d + 1) (zl/zh)^(d + 1))) NIntegrate[(x)/Sqrt[(1 - x^2) (1 - (((1 + t^2 (1 - (zl/zh)^(d + 1))^-1)^-1) (zl/zh)^-6) x^3)], {x, 0, (zl/zh)^2}, PrecisionGoal -> 6, Method -> "LocalAdaptive"]
tz[a_?NumericQ, zl_?NumericQ, zh_?NumericQ] := t /. FindRoot[toroot[a, t, zl, zh], {t, -15, -20, 0}, AccuracyGoal -> Automatic, AccuracyGoal -> 12, PrecisionGoal -> 12]
intSL1[a_?NumericQ, zl_?NumericQ, zh_?NumericQ] := ((-1/(d - 1)) (zl^(2 d) (1 + tz[a, zl, zh]^2 (1 - (zl/zh)^(d + 1))^-1))^-1 zl^(2 d))*NIntegrate[x^d ((1 - (zl/zh)^(d + 1) x^(d + 1))/(1 - (zl^(2 d) (1 + tz[a, zl, zh]^2 (1 - (zl/zh)^(d + 1))^-1))^-1 (zl x)^(2 d)))^(1/2), {x, 10^-8, 1}, MaxRecursion -> 20, PrecisionGoal -> 6, Method -> "GlobalAdaptive"]
intSL2[a_?NumericQ, zl_?NumericQ, zh_?NumericQ] := ((-(zl/zh)^(d + 1) (d + 1))/(2 (d - 1))) * NIntegrate[x ((1 - (zl^(2 d) (1 + tz[a, zl, zh]^2 (1 - (zl/zh)^(d + 1))^-1))^-1 (zl x)^(2 d))/(1 - (zl/zh)^(d + 1) x^(d + 1)))^(1/2), {x, 10^-8, 1}, MaxRecursion -> 20, PrecisionGoal -> 6, Method -> "GlobalAdaptive"]
intSL3[a_?NumericQ, zl_?NumericQ, zh_?NumericQ] := (zl/zh)^(d + 1) * NIntegrate[x/((1 - (zl/zh)^(d + 1) x^(d + 1)) (1 - (zl^(2 d) (1 + tz[a, zl, zh]^2 (1 - (zl/zh)^(d + 1))^-1))^-1 (zl x)^(2 d)))^(1/2), {x, 10^-8, 1}, MaxRecursion -> 20, PrecisionGoal -> 6, Method -> "GlobalAdaptive"]
functionSL[a_?NumericQ, zl_?NumericQ, zh_?NumericQ] := ((-((1 - (zl^(2 d) (1 + tz[a, zl, zh]^2 (1 - (zl/zh)^(d + 1))^-1))^-1 zl^(2 d)) (1 - (zl/zh)^(d + 1)))^(1/2)/(d - 1)) + intSL1[a, zl, zh] + intSL2[a, zl, zh] + intSL3[a, zl, zh] + 1)/(4 zl^(d - 1))


One issue is that every time I change zh I also need to change the parameter t in the FindRoot,

tz[a_?NumericQ, zl_?NumericQ, zh_?NumericQ] := t /. FindRoot[toroot[a, t, zl, zh], {t, -15, -20, 0}, AccuracyGoal -> Automatic, AccuracyGoal -> 12, PrecisionGoal -> 12]


I already know that the minimum always occurs around ~0.93*zh, an example would be, for zh=1 the minimum is at zl=0.930685. Now if you plot tz[0.01,zl,1] you will see the corresponding t you need to put in FindRoot for it to find a solution. You can see that zl=0.930685 is around t=-15 so that is why I set that as the starting point in FindRoot. The same goes for all other zh.

The code for finding the minimum for zh=1 is,

FindMinimum[functionSL[0.01, zl, 1], {zl, 0.8, 1}, AccuracyGoal -> 10, PrecisionGoal -> 10, MaxIterations -> 100] // Quiet
{0.216556, {zl -> 0.930685}}


If I do it for all zh at the same time I have,

In:= Zmin = Table[FindMinimum[{functionSL[0.01, zl, zh], zl <= zh}, {zl, 0.9, 0.8, 100}], {zh, {1, 5, 10, 50, 100}}] // Quiet // AbsoluteTiming

Out= {268.65, {{0.216556, {zl -> 0.930685}}, {0.00866033, {zl -> 4.653}}, {0.00216507, {zl -> 9.30576}}, {0.0000874471, {zl -> 44.7789}}, {0.0000326264, {zl -> 64.6718}}}}


You see that for smaller zh I got the correct values for zl but for higher zh like zh=100 you see that the result is not correct. Also you see that I used 0.9 as a starting point. The problem has to do with what I put in FindRoot, I think.

FindRoot[toroot[a, t, zl, zh], {t, -15, -20, 0}, AccuracyGoal -> Automatic, AccuracyGoal -> 12, PrecisionGoal -> 12]


putting {t, -15, -20, 0} makes it more likely that I find the correct roots for lower zh, but not so for higher.

How should I resolve this?

## 1 Answer

Without understanding your problem, I changed the initial conditions to {zl, .8 zh, 0.9 zh} and tried

Table[{zh, zl /. FindMinimum[functionSL[0.01, zl, zh],
{zl, .8 zh, 0.9 zh}][]}
, {zh, {.8,1.1, 10, 20, 100}}] // Quiet
(*{{0.8, 0.744584}, {1.1, 1.02374}, {10, 9.3059}, {20, 18.6118}, {100, 93.059}}*)


which gives tabeled result {zh,zl}up to zh=100.

• Could you explain why changing the range in FindMinimum as you wrote works as compared to what I used which is the beginner's way. – mathemania Dec 9 '20 at 18:16
• The starting values for zl in FindMinimum now depend on zh which forces a solution zl near zh (perhaps). – Ulrich Neumann Dec 9 '20 at 21:21
• Thank you very much! Did not know it was that simple. – mathemania Dec 10 '20 at 7:08