int i , n = 8;
for (i = 2; i <= n; i=pow(i,2)) {
cout << “Hello World !!!\n”;
}
time complexity is simply how many times your code runs depending on input ( over simplification, assuming instructions taking unit time).
so for your code
it iterates like this
2^2 , 2^4 , 2^8 , … 2^(2^n)
where n is how many times it runs
=> 2^(2^n)= N
=> log(N)=2^n
=> n = log(log(N))
( => has no specific meaning here , you could though think as implies if you do)
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Thanks a lot 
. Nice explanation 