718531 is divisible by:
7
3
11
5
Sum of digits at odd places = 7 + 8 + 3 = 18
Sum of digits at even places = 1 + 5 + 1 = 7
Difference = 18 - 7 = 11
Since, 11 is divisible by 11.
Hence, 718531 is divisible by 11.
If P(k) is the statement 23k - 1 is divisible by 7, then P(k + 1) is