Summation loop (sigma notation) help

Please help! I am slowly but surely understanding Matlab, but struggle a bit every now and then. I am trying to make a summation loop out of:

a=1:100; n=2; sum_a=0.5; for ii=1:length(a); sum_a=sum_a + a(ii).^n; end display(sum_a);

The way i read it is that (a)^n + (a+a)^n + (a+a+a)^n....

Is this correct?

Many thanks :D

Answer 1

[ES]

What yours does is sum of squares of natural numbers from 1 to 1000 +0.5. That is 0.5+1^2+2^2+3^2+...+1000^2;

Is this what you want?

[S]

No i want the 0.5 to be the first number and then it goes up in powers of 2 till infinite ;/

Answer 2

[Mischa Kim]

In your code

 sum_a = 0.5; 
 for ii = 1:length(a)
    sum_a = sum_a + a(ii).^n; 
  end

you initially set sum_a = 0.5. In the loop you then add a(ii).^n to the previously stored value of sum_a. In other words:

ii = 1:  sum_a = 0.5 + 1^2      ... which equals 1.5
ii = 2:  sum_a = 1.5 + 2^2      ... which equals 5.5 
ii = 3:  sum_a = 5.5 + 3^2      ... which equals 14.5 
...

[S]

Thanks :D

Answer 3

[Iain]

If you want: (a)^n + (a+a)^n + (a+a+a)^n....

What you actually want is:

 a = 0.5; % or whatever
 sum = 0;
 n = 2;
 for i = 1:m % m being the limit
  sum = sum + (i*a)^n
 end

So, if m is 0, you get sum = 0, if m is 1, you get 0.25, if m is 2, sum = 1.25, m = 3, sum = 3.5, m = 4 sum = 7.5 ...

[S]

Thanks this helps alot :D