PROBLEM
Let g(n) be a sequence defined as follows:
g(4) = 13,
g(n) = g(n-1) + gcd(n, g(n-1)) for n > 4. gcd is great common divisor.
The first few values are:
n 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 ...
g(n) 13 14 16 17 18 27 28 29 30 31 32 33 34 51 54 55 60 ...
You are given that g(1000) = 2524 and g(1000 000) = 2624152.
Find g(10^15).
HOW CAN I SOLVE THIS PROBLEM. PLEASE HELP ME!!