I have the code which generates an array for a1. For N = 10, what does a1 = [a1 a1+N/2] stores.? Is there any need?

if mod(N,2)==0 && mod(N,4)~= 0
  a1 = 1: (N/2-1);
  a1 = a1(gcd(a1,N/2)==1);
  a1 = [a1 a1+N/2];
end

Answer 1

[Walter Roberson]

a1 = [a1 a1+N/2]

is the same as

a1 = horzcat(a1, a1+N/2);

which is to say, append a1+N/2 on the end of the current a1