We do not sample the function enough to get an accurate portrayal of the signal. 
I changed the sample rate to take 25 samples per period.

F0 =13;
T0 =1/F0;
Ts = 1/(F0*25);
t  = 0:Ts:13*T0;
x = real(exp(j*(2*pi*F0*t-pi/2)));
plot(t,x)

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Abstract algebra continues the conceptual developments of linear algebra, on an even grander scale.

Dr. Paul Garrett