It looks as though you want to find the largest divisor of n less than n if n is not a prime number, and otherwise the prime number n itself. You could perhaps make your code more efficient if you replaced "i≤n/2" with "i<=sqrt(n)".
I am not sure about the name of your function. I don't understand that request about "plotting the number of earring".
function pgd = entier(n)
i = 2;
while i≤n/2 & mod(n,i)~-0
i = i+1;
end
if mod(n,i)==0
pgd = n/i;
else
pgd = n;
end
end
