Trang chủ » Expected Value of the Maximum Number of Heads in n Flips

# Expected Value of the Maximum Number of Heads in n Flips

$\begingroup$ Suppose we compute the generating function of binary strings having at most $q$ straight heads. There are four cases, according to whether the string starts with heads or tails and ends with heads or tails. We get  G_ { HH } ( omega ) = z\frac { 1-z^ { q } } { 1-z } \sum_ { k=0 } ^\infty \left ( \frac { omega } { 1-z } z\frac { 1-z^ { q } } { 1-z } \right ) ^k. 
Continuing we get  G_ { HT } ( omega ) = G_ { HH } ( omega ) \frac { z } { 1-z }. 

furthermore  G_ { TT } ( omega ) = \frac { omega } { 1-z } \sum_ { k=0 } ^\infty \left ( z\frac { 1-z^ { q } } { 1-z } \frac { z } { 1-z } \right ) ^k. 
ultimately we have  G_ { TH } ( z ) = G_ { TT } ( z ) z\frac { 1-z^ { q } } { 1-z }. 
The sum term is  \frac { 1 } { 1-z^2 ( 1-z^q ) / ( 1-z ) ^2 } = \frac { 1-2z+z^2 } { 1-2z+z^2-z^2 ( 1-z^q ) } = \frac { 1-2z+z^2 } { 1-2z+z^ { q+2 } }. 
The gene on this is  z\frac { 1-z^ { q } } { 1-z } \left ( 1+\frac { z } { 1-z } \right ) + \frac { z } { 1-z } \left ( 1+z\frac { 1-z^ { q } } { 1-z } \right )  which is  z\frac { 1-z^ { q } } { ( 1-z ) ^2 } + \frac { z } { ( 1-z ) ^2 } ( 1-z^ { q+1 } ) = \frac { 2z-z^ { q+1 } -z^ { q+2 } } { ( 1-z ) ^2 }. 
Multiplying we obtain the generating function  G_q ( z ) = \frac { 2z-z^ { q+1 } -z^ { q+2 } } { 1-2z+z^ { q+2 } }. 
It follows that the expectation time $2^n$ is given by

 [ z^n ] \left ( 0\times G_0 ( z ) + \sum_ { q=1 } ^n q ( G_q ( z ) -G_ { q-1 } ( omega ) ) \right ). 
The sum simplifies to  \sum_ { q=1 } ^n q G_q ( z ) – \sum_ { q=0 } ^ { n-1 } ( q+1 ) G_q ( z ) = \sum_ { q=0 } ^n q G_q ( z ) – \sum_ { q=0 } ^ { n-1 } ( q+1 ) G_q ( omega ) \\ = normality G_n ( omega ) – \sum_ { q=0 } ^ { n-1 } G_q ( z ). 
and hence the expectation is  \frac { 1 } { 2^n } [ z^n ] \left ( nitrogen G_n ( omega ) – \sum_ { q=0 } ^ { n-1 } G_q ( omega ) \right ). 
This gives the sequence  1/2,1, { \frac { 11 } { 8 } }, { \frac { 27 } { 16 } }, { \frac { 31 } { 16 } }, { \frac { 69 } { 32 } }, { \frac { 75 } { 32 } }, { \frac { 643 } { 256 } }, { \frac { 1363 } { 512 } }, { \frac { 1433 } { 512 } }, \ldots 
Multiplying by $2^n$ we obtain  1, 4, 11, 27, 62, 138, 300, 643, 1363, 2866, \ldots  which is OEIS A119706 where the above calculation is confirmed .

The following Maple code can be used to explore these generating functions. The procedure v computes the generating function of the maximal run distance of a string of $n$ bits by total count. The routine w computes it from the generating serve $G_q ( omega ).$

v :=
proc(n)
option remember;
local gf, k, d, mxrun, len;

gf := 0;

for k from 2^n to 2^(n+1)-1 do
d := convert(k, base, 2);

mxrun := 0;
for pos to n do
if d[pos] = 1 then
len := 1;

pos := pos+1;
while pos <= n do
if d[pos] = 1 then
len := len+1;
pos := pos+1;
else
break;
fi;
od;

if len>mxrun then
mxrun := len;
fi;
fi;
od;

gf := gf  + z^mxrun;
od;

gf;
end;

G := q -> (2*z-z^(q+1)-z^(q+2))/(1-2*z+z^(q+2));

w :=
proc(n)
option remember;
local gf, mxrun;

gf := 1;

for mxrun to n do
gf := gf +
coeftayl(G(mxrun)-G(mxrun-1), z=0, n)*z^mxrun;
od;

gf;
end;

X := n -> coeftayl(n*G(n)-add(G(q), q=0..n-1), z=0, n)/2^n;


here are two examples .

> v(4);
4      3      2
z  + 2 z  + 5 z  + 7 z + 1

> w(4);
4      3      2
z  + 2 z  + 5 z  + 7 z + 1


Addendum. Responding to the question of the OP, the maximal test length distribution for $n=50$ is

> w(50);
50      49      48       47       46       45        44        43        42
z   + 2 z   + 5 z   + 12 z   + 28 z   + 64 z   + 144 z   + 320 z   + 704 z

41         40         39          38          37          36
+ 1536 z   + 3328 z   + 7168 z   + 15360 z   + 32768 z   + 69632 z

35           34           33            32            31
+ 147456 z   + 311296 z   + 655360 z   + 1376256 z   + 2883584 z

30             29             28             27              26
+ 6029312 z   + 12582912 z   + 26214400 z   + 54525952 z   + 113246208 z

25              24               23               22
+ 234881024 z   + 486539259 z   + 1006632909 z   + 2080374408 z

21               20                19                18
+ 4294964912 z   + 8858356224 z   + 18253535488 z   + 37580568576 z

17                 16                 15                 14
+ 77307408384 z   + 158903894017 z   + 326369607799 z   + 669786836360 z

13                  12                  11
+ 1373319005440 z   + 2812533538048 z   + 5749650288420 z

10                   9                   8
+ 11716183298140 z   + 23723022576779 z  + 47402584528885 z

7                    6                    5
+ 92066138963408 z  + 168050756947888 z  + 267156803852044 z

4                    3                   2
+ 310228979841119 z  + 174887581402185 z  + 19394019617001 z

+ 32951280098 z + 1

source : https://ontopwiki.com
Category : Finance