My code is giving me a straight line for values of N other than 5

clear variables
close all
M = 250; % The length of each function
t = -5:0.1:5;
y = zeros(1,M); % Initiallize the sum to zero
N = 2;
for k=N
 ck = (cos(k*pi)-1);
 b= (2/((k*pi)^2))*(cos(k*pi*t));
 syms k 
y= symsum(0.5+(ck*b),k,1,N); % computing the sum
end
plot(t,y,'linewidth',2,'color','m')
a= title('x(t)');
set(a,'fontsize',14);
a= xlabel('x');
set(a,'fontsize',20);
a = ylabel('y');
set(a,'fontsize',20);
a = zlabel('z');
set(a,'fontsize',20);
grid;

Answer 1

[matt]

Define k as symbolic before defining b and ck. In your posted code, when the for loop is entered, k=N=2 so ck = (cos(2*pi)-1)=0 and b=(2/((2*pi)^2))*(cos(2*pi*t)). when symsum is reached, syms k is not defined in the function 0.5+(ck*b).

best, matt

clear variables
close all
M = 250; % The length of each function
t = -5:0.1:5;
y = zeros(1,M); % Initiallize the sum to zero
N = 2;
for k=N
 syms k 
 ck = (cos(k*pi)-1);
 b= (2/((k*pi)^2))*(cos(k*pi*t));
y= symsum(0.5+(ck*b),k,1,N); % computing the sum
end

plot(t,y,'linewidth',2,'color','m')
a= title('x(t)');
set(a,'fontsize',14);
a= xlabel('x');
set(a,'fontsize',20);
a = ylabel('y');
set(a,'fontsize',20);
a = zlabel('z');
set(a,'fontsize',20);
grid;

Answer 2

[Torsten]

I'm quite sure that the 0.5 has to be taken out of the symsum:

syms k N integer
syms t
y = 0.5 + symsum((cos(k*pi)-1)*(2/((k*pi)^2))*(cos(k*pi*t)),k,1,N)

y = 

tnum = -5:0.01:5;
Nnum = 2;
ynum = arrayfun(@(tnum)double(subs(y,[N t],[Nnum tnum])),tnum);
plot(tnum,ynum)

Answer 3

[VBBV]

clear variables
close all
M = 250; % The length of each function
t = -5:0.1:5;
y = zeros(1,M); % Initiallize the sum to zero
N = 2;
syms k  
  
for n=1:N
 
    ck = (cos(k*pi)-1);
    b= (2/((k*pi)^2))*(cos(k*pi*t));    
    y= symsum(0.5+(ck*b),k,1,n); % computing the sum
    plot(t,y,'linewidth',2)
    hold on
 
end
legend('N=1','N=2','location','best')
a= title('x(t)');
set(a,'fontsize',14);
a= xlabel('x');
set(a,'fontsize',20);
a = ylabel('y');
set(a,'fontsize',20);
a = zlabel('z');
set(a,'fontsize',20);
grid;