# How to find the global maximum of a function? [closed]

I tried the following but plotting the function clearly reveals that there is a global maximum around $r = 7$ (here $a = 0.53$)

FindMaximum [{r^2*(Exp[-(r)/(3 a)]* (27 a^2 - 18 a r + 2 r^2)/(9 a^2))^2 ,
0 <= r < Infinity}  , {r, 0}]


{0.253131, {r -> 0.39222}}

I tried NMaximum, Max, MaxValue, Maximize, FindMaxValue, FindMaximum but nothing works!

(..I wonder why there are so many maxima finding functions anyway!..)

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## closed as too localized by Oleksandr R., Silvia, Sjoerd C. de Vries, m_goldberg, Yves KlettApr 30 '13 at 6:34

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Why don't you do it the calculus way: find those r's that zero the first derivative of the function? –  Spawn1701D Apr 19 '13 at 2:36
"(...I wonder why there are so many maxima finding functions anyway!...)" - because some of the functions are intended for local optima, and some of them attempt to find global optima. –  Ｊ. Ｍ. Apr 19 '13 at 2:47
possible duplicate of How to find all the local minima/maxima in a range –  Silvia Apr 29 '13 at 18:50

According to the documentation, don't use floating point numbers when finding global maxima:

a = 53/100;
Maximize[{r^2*(Exp[-(r)/(3 a)]*(27 a^2 - 18 a r + 2 r^2)/(9 a^2))^2,
0 <= r < Infinity}, r] // N

(* {1.73265, {r -> 6.92924}} *)

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Thanks! But why is this "//N" important? I am getting some "garbage" if I remove that. –  user6818 Apr 19 '13 at 3:23
It's not garbage, it's the exact symbolic solution. The //N converts the exact solution to a floating point number. –  Mike Z. Apr 19 '13 at 3:27
Z I get this - (how is this a "symbolic solution") - {(1/639128961) E^(-(200/159) Root[-12059037 + 37921500 #1 - 19080000 #1^2 + 2000000 #1^3 &, 3]) Root[-12059037 + 37921500 #1 - 19080000 #1^2 + 2000000 #1^3 &, 3]^2 (75843 - 95400 Root[-12059037 + 37921500 #1 - 19080000 #1^2 + 2000000 #1^3 &, 3] + 20000 Root[-12059037 + 37921500 #1 - 19080000 #1^2 + 2000000 #1^3 &, 3]^2)^2, {r -> Root[-12059037 + 37921500 #1 - 19080000 #1^2 + 2000000 #1^3 &, 3]}} –  user6818 Apr 21 '13 at 21:37
Also how can I tweak it to tell me all the local maxima and that as a function of "a". Without having to assign any particular value of "a"? (just that a is always some real number) –  user6818 Apr 21 '13 at 21:39