@Carl Love 

Thanks, now it is correct. 

Thanks Tom and Carl!

@Carl Love 

I got an error in the Mysum. ( I used s2)

Error, (in s2) invalid input: sum uses a 2nd argument, k, which is missing

@tomleslie 

I get an error in mySum (I used s2)

Error, (in s2) invalid input: sum uses a 2nd argument, k, which is missing

I tried "add" as well. But was not the solution.

@Carl Love 

The limits are: j=0 to n

And I need to verify for n= 1, n=2, n=3 and n=4.

@vv 

Thanks that was helpful. I now get the solutions for a,b and c. Even without using a set.