sub primes_opt_till_79 {
    my $n = shift;
    return if $n < 2;

    my @primes = (2);
    for my $i (3 .. $n) {
				if($i%2==0){next}
				if($i%3==0){next}
				if($i%5==0){next}
				if($i%7==0){next}
				if($i%11==0){next}
				if($i%13==0){next}
				if($i%17==0){next}
				if($i%19==0){next}
				if($i%23==0){next}
				if($i%29==0){next}
				if($i%31==0){next}
				if($i%37==0){next}
				if($i%41==0){next}
				if($i%43==0){next}
				if($i%47==0){next}
				if($i%53==0){next}
				if($i%59==0){next}
				if($i%61==0){next}
				if($i%67==0){next}
				if($i%71==0){next}
				if($i%73==0){next}
				if($i%79==0){next}

        my $sqrt = sqrt $i;
        my $notprime;
        for my $p (@primes) {
            last if $p > $sqrt;

            $notprime = 1, last if 0 == $i % $p;
        }
        push @primes, $i unless $notprime;
    }
    return @primes
}