What is the 1 0 0 0 th digit to the right of the decimal point in the decimal representation of ( 1 + 2 ) 3 0 0 0 ?
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Just finished posting this and realized there is a cute alternative way to see that y < 1 0 − 1 0 0 0 As I pointed out, x and y are multiplicative inverses. So it suffices to prove that x > 1 0 1 0 0 0 But we know that x ≈ x + y = 2 i = 0 ∑ 1 5 0 0 ( 2 i 3 0 0 0 ) 2 i If we pair the first and last terms, second and second-last, and so on, we find that the middle pair has the smallest average, namely 2^{750}, so x + y > 2 i = 0 ∑ 1 5 0 0 ( 2 i 3 0 0 0 ) 2 7 5 0 . Finally, since the even binomial coefficients have the same sum as the odd ones, namely \frac{1}{2} 2^[3000} we have = 2 3 7 5 0 and it is easy to see 2 3 . 7 5 > 1 0 . Using a calculator, I find ( 1 + 2 ) 3 ≈ 2 3 . 8 1 4 6 so this wasn't such a bad approximation.
Funny that you can't add a solution if you didn't get the right answer...
Consider this recurrence relation:
a n = 2 ⋅ a n − 1 + a n − 2 a 0 = 2 a 1 = 2
The solution is: a n = ( 1 + 2 ) n + ( 1 − 2 ) n .
(I didn't know about Pisot–Vijayaraghavan numbers, but the characteristic polynomial of the recurrence relation above and the minimal polynomial of 1 + 2 are the same and that's the link between Pisot–Vijayaraghavan numbers and recurrence relations).
We are interested in a 3 0 0 0 . By definition, this is an integer, and it equals a n = ( 1 + 2 ) 3 0 0 0 + ( 1 − 2 ) 3 0 0 0 .
To estimate the second term: ( 1 − 2 ) 3 0 0 0 = ( 2 − 1 ) 3 0 0 0 = 1 0 l o g 1 0 ( ( 2 − 1 ) 3 0 0 0 ) = 1 0 3 0 0 0 ⋅ l o g 1 0 ( 2 − 1 ) ≈ 1 0 − 1 1 4 8 . 3 2 7 0 5 6 0 1 3 5 8 9 ≈ 1 0 − 1 1 4 9 ⋅ 1 0 0 . 6 7 2 9 4 3 9 8 6 4 1 1 ≈ 4 . 7 0 9 1 6 5 8 5 5 2 ⋅ 1 0 − 1 1 4 9 .
With the recurrence relation, we can calculate with a computer that:
a 3 0 0 0 = 2 1 2 3 5 1 8 3 2 7 3 5 6 6 1 2 1 1 8 5 0 0 3 6 7 2 1 3 7 5 8 8 6 7 7 6 5 4 1 4 1 9 7 4 1 8 4 6 8 3 0 7 9 8 0 4 7 7 9 7 6 4 9 3 2 6 6 3 3 8 3 0 2 6 5 8 0 0 3 4 8 7 5 4 9 2 2 7 2 6 6 1 1 4 8 2 3 2 9 6 9 6 9 7 8 3 8 6 9 4 1 7 1 2 3 8 6 6 2 8 3 1 3 2 3 3 7 2 2 3 2 9 3 9 6 8 3 1 7 2 9 2 6 2 9 7 6 2 7 1 9 3 5 9 1 3 0 2 8 9 6 8 6 8 6 2 1 7 8 5 4 1 7 1 6 9 4 4 7 9 7 5 0 9 5 0 0 0 7 1 3 8 5 9 8 7 0 2 0 4 7 8 7 4 9 2 8 9 3 7 0 0 6 0 4 8 8 2 5 9 4 1 7 2 8 8 0 4 7 2 6 4 4 5 5 5 8 4 7 2 9 5 0 5 9 5 4 8 4 3 7 2 4 0 8 2 5 8 8 0 5 2 5 5 9 7 3 9 6 3 8 9 3 6 1 2 5 6 5 7 4 4 6 5 9 4 5 2 5 8 8 8 8 5 6 3 4 6 7 2 2 5 1 1 2 4 6 9 4 9 6 1 2 6 7 8 0 4 8 9 8 1 5 2 7 9 5 8 1 5 1 0 2 3 8 3 2 8 4 9 4 7 0 7 3 0 2 1 1 5 8 6 2 3 2 6 8 9 7 9 1 6 6 7 0 8 8 9 7 3 7 1 7 1 6 2 8 8 8 1 3 3 2 8 5 7 4 1 2 5 8 9 6 0 0 2 1 0 5 5 2 2 3 4 2 5 3 1 2 6 4 5 0 0 6 6 7 6 6 6 0 1 0 5 9 3 0 0 4 8 0 5 6 4 1 2 0 1 4 0 7 4 8 5 5 2 3 3 6 9 6 6 1 1 2 3 2 7 0 3 0 9 3 2 1 7 2 1 3 1 9 9 9 0 4 2 5 6 0 9 5 8 6 2 1 5 8 9 1 2 1 6 3 7 9 5 2 3 1 0 4 0 2 7 1 0 5 1 5 4 3 2 8 2 3 0 7 1 5 9 4 3 0 2 8 6 8 8 4 2 7 9 6 4 5 0 4 7 3 7 0 1 6 7 4 0 7 1 9 7 0 5 6 4 8 0 0 4 7 6 0 0 3 3 4 7 7 4 3 9 0 6 0 1 2 0 8 2 2 0 5 6 4 0 1 8 5 6 8 4 2 6 6 4 8 4 3 4 9 8 2 6 4 7 5 7 7 3 2 1 9 6 9 3 6 5 7 9 3 8 0 2 0 6 4 1 8 7 2 0 9 5 2 0 0 6 8 6 5 6 1 2 5 6 2 2 1 0 4 1 2 2 0 7 8 2 8 1 3 1 4 5 1 6 1 6 3 6 5 2 9 5 8 9 8 2 3 4 4 0 2 5 3 7 3 3 6 7 6 2 3 2 0 3 8 2 5 0 8 7 5 4 9 2 1 2 0 2 8 2 5 7 5 4 8 0 7 4 0 4 5 5 7 8 2 0 0 0 7 0 5 5 1 2 1 1 0 2 6 4 5 1 5 6 8 4 2 1 2 6 0 6 4 5 2 6 3 0 1 5 5 0 0 3 3 5 0 3 7 1 0 3 2 6 2 4 3 5 8 0 6 8 6 2 0 2 3 4 3 6 8 9 2 9 0 3 8 5 6 1 2 8 5 8 0 1 3 2 0 8 2 9 2 4 7 9 0 4 8 8 9 2 3 2 4 2 4 2 8 9 1 4 3 7 9 2 6 9 3 6 3 7 9 5 7 9 7 7 1 6 6 1 2 7 1 2 1 9 6 7 7 0 6 4 3 9 6 1 2 0 7 3 9 8 0 3 3 0 9 2 3 0 5 0 2 9 8 1 3 5 5 6 8 8 4 8 4 9 0 7 1 3 2 4 7 1 1 3 6 0 2 3 8 5 4 1 0 1 0 1 3 7 0 6 5 8 0 2 4 3 1 1 5 3 6 2 7 3 8 6 4 7 9 3 9 7 4 8 1 2 3 3 4 8 6 8 4 8 3 1 3 5 7 2 4 4 2 7 4 1 3 6 4 5 8 3 7 8 9 0 8 3 5 7 2 5 5 9 5 8 4 9 1 9 2 5 9 8 0 3 5 6 5 8 8 7 4 3 8 7 6 4 9 7 7 7 9 2 1 9 6 3 6 4 0 7 8 7 3 2 8 5 9 4 5 2 0 2 0 1 6 3 5 9 3 0 7 2 9 8 9 8 5 4 1 2 9 0 1 5 0 4 4 8 9 9 7 5 2 8 0 7 4 3 1 8 2 1 8 6 4 9 8 4 9 0 5 7 9 8 2 4 0 5 7 3 4 9 1 4 0 6 0 0 7 4 1 1 5 0 8 0 7 1 2 6 8 9 2 5 5 4 3 4 1 8 3 8 2 4 5 5 8 3 5 1 5 5 3 2 4 5 0 0 0 0 0 0 2 .
So:
( 1 + 2 ) 3 0 0 0 ≈ a 3 0 0 0 − 4 . 7 0 9 1 6 5 8 5 5 2 ⋅ 1 0 1 1 4 9 = 2 1 2 3 5 1 8 3 2 7 3 5 6 6 1 2 1 1 8 5 0 0 3 6 7 2 1 3 7 5 8 8 6 7 7 6 5 4 1 4 1 9 7 4 1 8 4 6 8 3 0 7 9 8 0 4 7 7 9 7 6 4 9 3 2 6 6 3 3 8 3 0 2 6 5 8 0 0 3 4 8 7 5 4 9 2 2 7 2 6 6 1 1 4 8 2 3 2 9 6 9 6 9 7 8 3 8 6 9 4 1 7 1 2 3 8 6 6 2 8 3 1 3 2 3 3 7 2 2 3 2 9 3 9 6 8 3 1 7 2 9 2 6 2 9 7 6 2 7 1 9 3 5 9 1 3 0 2 8 9 6 8 6 8 6 2 1 7 8 5 4 1 7 1 6 9 4 4 7 9 7 5 0 9 5 0 0 0 7 1 3 8 5 9 8 7 0 2 0 4 7 8 7 4 9 2 8 9 3 7 0 0 6 0 4 8 8 2 5 9 4 1 7 2 8 8 0 4 7 2 6 4 4 5 5 5 8 4 7 2 9 5 0 5 9 5 4 8 4 3 7 2 4 0 8 2 5 8 8 0 5 2 5 5 9 7 3 9 6 3 8 9 3 6 1 2 5 6 5 7 4 4 6 5 9 4 5 2 5 8 8 8 8 5 6 3 4 6 7 2 2 5 1 1 2 4 6 9 4 9 6 1 2 6 7 8 0 4 8 9 8 1 5 2 7 9 5 8 1 5 1 0 2 3 8 3 2 8 4 9 4 7 0 7 3 0 2 1 1 5 8 6 2 3 2 6 8 9 7 9 1 6 6 7 0 8 8 9 7 3 7 1 7 1 6 2 8 8 8 1 3 3 2 8 5 7 4 1 2 5 8 9 6 0 0 2 1 0 5 5 2 2 3 4 2 5 3 1 2 6 4 5 0 0 6 6 7 6 6 6 0 1 0 5 9 3 0 0 4 8 0 5 6 4 1 2 0 1 4 0 7 4 8 5 5 2 3 3 6 9 6 6 1 1 2 3 2 7 0 3 0 9 3 2 1 7 2 1 3 1 9 9 9 0 4 2 5 6 0 9 5 8 6 2 1 5 8 9 1 2 1 6 3 7 9 5 2 3 1 0 4 0 2 7 1 0 5 1 5 4 3 2 8 2 3 0 7 1 5 9 4 3 0 2 8 6 8 8 4 2 7 9 6 4 5 0 4 7 3 7 0 1 6 7 4 0 7 1 9 7 0 5 6 4 8 0 0 4 7 6 0 0 3 3 4 7 7 4 3 9 0 6 0 1 2 0 8 2 2 0 5 6 4 0 1 8 5 6 8 4 2 6 6 4 8 4 3 4 9 8 2 6 4 7 5 7 7 3 2 1 9 6 9 3 6 5 7 9 3 8 0 2 0 6 4 1 8 7 2 0 9 5 2 0 0 6 8 6 5 6 1 2 5 6 2 2 1 0 4 1 2 2 0 7 8 2 8 1 3 1 4 5 1 6 1 6 3 6 5 2 9 5 8 9 8 2 3 4 4 0 2 5 3 7 3 3 6 7 6 2 3 2 0 3 8 2 5 0 8 7 5 4 9 2 1 2 0 2 8 2 5 7 5 4 8 0 7 4 0 4 5 5 7 8 2 0 0 0 7 0 5 5 1 2 1 1 0 2 6 4 5 1 5 6 8 4 2 1 2 6 0 6 4 5 2 6 3 0 1 5 5 0 0 3 3 5 0 3 7 1 0 3 2 6 2 4 3 5 8 0 6 8 6 2 0 2 3 4 3 6 8 9 2 9 0 3 8 5 6 1 2 8 5 8 0 1 3 2 0 8 2 9 2 4 7 9 0 4 8 8 9 2 3 2 4 2 4 2 8 9 1 4 3 7 9 2 6 9 3 6 3 7 9 5 7 9 7 7 1 6 6 1 2 7 1 2 1 9 6 7 7 0 6 4 3 9 6 1 2 0 7 3 9 8 0 3 3 0 9 2 3 0 5 0 2 9 8 1 3 5 5 6 8 8 4 8 4 9 0 7 1 3 2 4 7 1 1 3 6 0 2 3 8 5 4 1 0 1 0 1 3 7 0 6 5 8 0 2 4 3 1 1 5 3 6 2 7 3 8 6 4 7 9 3 9 7 4 8 1 2 3 3 4 8 6 8 4 8 3 1 3 5 7 2 4 4 2 7 4 1 3 6 4 5 8 3 7 8 9 0 8 3 5 7 2 5 5 9 5 8 4 9 1 9 2 5 9 8 0 3 5 6 5 8 8 7 4 3 8 7 6 4 9 7 7 7 9 2 1 9 6 3 6 4 0 7 8 7 3 2 8 5 9 4 5 2 0 2 0 1 6 3 5 9 3 0 7 2 9 8 9 8 5 4 1 2 9 0 1 5 0 4 4 8 9 9 7 5 2 8 0 7 4 3 1 8 2 1 8 6 4 9 8 4 9 0 5 7 9 8 2 4 0 5 7 3 4 9 1 4 0 6 0 0 7 4 1 1 5 0 8 0 7 1 2 6 8 9 2 5 5 4 3 4 1 8 3 8 2 4 5 5 8 3 5 1 5 5 3 2 4 5 0 0 0 0 0 0 1 . 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 5 2 9 0 8 3 4 1 4 4 8 . . . .
To get more digits, we can remark that
( 1 + 2 ) ( 1 − 2 ) = − 1 ⇒ ( 1 + 2 ) 3 0 0 0 ( 1 − 2 ) 3 0 0 0 = 1 ⇒ ( 1 − 2 ) 3 0 0 0 = 1 / ( 1 + 2 ) 3 0 0 0 ≈ 1 / a 3 0 0 0 .
The fractional part is then 1 − 1 / a 3 0 0 0 (the 1 is borrowed from a 3 0 0 0 ). If we want to use only arbitrary precision integer arithmetic to calculate the digits, instead calculate 1 0 k − 1 0 k / a 3 0 0 0 . Because we know the first digit after the decimal point is not 0 (it is 9), the number we calculated can be put right after the decimal point. The last digit we find is likely wrong though, so we leave it off. These are the first 10000 digits after the decimal point:
0 . 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 5 2 9 0 8 3 4 1 4 4 8 3 7 2 8 4 2 8 4 9 3 9 9 9 5 0 0 2 8 5 7 1 6 0 0 7 0 2 9 7 6 8 2 4 6 0 4 6 2 2 3 6 4 2 9 4 2 7 7 9 5 4 7 4 0 5 7 3 3 2 8 9 8 2 8 4 1 2 5 4 6 0 4 6 1 1 6 0 8 3 8 1 8 0 5 3 0 2 5 3 1 9 8 9 8 8 9 6 3 1 3 1 0 8 0 0 9 2 1 1 4 5 9 5 4 9 2 2 2 9 5 9 9 2 3 0 1 7 5 6 0 0 7 3 6 0 5 5 4 0 8 5 4 8 8 9 5 5 6 9 6 7 3 1 7 1 1 2 2 6 9 5 4 9 0 6 8 5 2 9 5 1 9 1 1 8 3 7 0 4 8 4 6 8 5 3 6 1 4 4 5 9 6 0 5 0 7 4 9 1 9 1 0 5 1 1 8 4 6 8 8 6 9 7 3 3 6 3 5 9 5 8 3 5 0 1 2 8 7 0 6 3 2 9 0 6 7 6 2 1 6 6 4 5 1 6 6 3 5 1 1 5 7 5 4 3 2 6 6 6 2 6 3 8 8 1 1 6 8 7 6 5 8 5 7 1 9 7 3 1 3 3 1 1 9 7 0 5 2 2 3 1 8 6 8 7 5 4 0 2 3 0 6 4 1 1 9 4 2 6 0 0 6 9 3 6 0 3 4 2 9 8 1 4 9 5 7 4 5 5 1 3 7 7 6 6 4 8 7 9 6 4 9 8 7 1 8 2 1 9 0 1 2 6 0 1 4 9 6 8 1 6 4 5 6 8 4 9 7 5 7 5 9 3 2 7 3 6 8 3 1 4 6 5 1 2 2 3 6 2 1 2 5 4 5 0 0 5 3 7 9 5 1 2 0 8 9 6 5 1 0 0 7 9 2 4 3 1 4 6 6 4 0 9 9 7 9 7 8 2 1 3 9 8 8 2 1 8 2 8 5 0 2 4 7 0 5 9 0 1 7 8 4 4 4 5 0 9 4 0 6 5 5 0 3 5 9 7 9 4 8 1 7 8 1 3 5 0 7 2 0 2 1 1 1 6 2 4 6 5 4 7 7 0 5 5 6 8 0 0 6 1 3 3 3 8 4 5 5 0 2 9 2 1 5 8 9 0 6 7 1 2 5 5 9 3 4 9 7 4 2 0 7 9 6 4 2 4 1 6 6 8 1 9 5 0 1 8 4 4 5 9 2 4 6 6 2 9 5 9 9 6 5 5 6 2 1 9 6 2 9 6 7 3 3 8 4 4 9 6 1 7 3 8 3 8 1 6 7 8 4 8 6 9 2 5 4 2 5 4 3 2 0 7 2 2 7 6 9 5 0 6 8 2 1 0 0 0 3 2 1 9 7 8 4 0 2 2 8 8 6 9 2 6 0 4 5 7 6 9 6 0 9 9 0 2 6 7 4 9 9 1 3 4 4 9 8 2 7 0 8 1 0 4 7 4 1 4 3 7 1 4 3 3 2 6 3 8 6 8 2 9 8 3 9 5 9 5 6 7 9 7 4 5 5 2 6 1 7 2 5 4 6 9 5 3 7 2 8 4 5 0 5 9 6 5 1 9 4 0 0 0 6 1 3 0 9 0 0 3 1 4 6 3 9 1 5 5 8 8 1 3 7 0 1 1 6 6 2 2 3 8 0 3 8 5 4 5 5 5 2 5 4 2 7 4 1 7 4 2 2 8 9 9 3 2 1 8 7 1 4 4 7 1 8 9 2 9 0 5 1 5 5 2 3 8 7 7 7 3 3 1 7 5 8 5 9 3 7 6 4 8 9 0 4 0 1 8 0 5 7 4 7 8 2 4 4 4 4 2 5 4 5 4 2 0 2 2 5 0 6 7 8 2 1 1 4 3 0 0 9 8 9 3 7 2 2 9 8 1 3 4 9 2 6 9 3 4 9 7 7 2 2 9 3 8 0 1 6 2 4 7 1 6 4 5 8 4 5 3 8 5 5 5 9 0 1 9 7 8 5 0 4 1 3 0 4 7 4 0 6 6 8 7 7 5 7 3 5 4 6 2 5 8 1 7 1 5 9 6 0 4 0 2 9 5 3 4 9 3 3 3 1 8 9 2 8 7 8 8 9 3 1 8 5 9 5 3 9 3 9 5 0 0 4 9 3 9 9 9 7 1 4 5 9 0 8 9 7 9 1 8 2 1 4 2 8 2 0 1 7 1 1 6 5 7 3 1 0 9 9 4 3 2 7 7 3 1 1 9 9 0 9 2 2 6 1 5 4 1 1 8 7 2 7 2 5 5 0 6 2 2 6 3 3 3 0 8 7 8 0 4 7 4 9 2 3 6 0 1 8 0 1 1 5 9 7 9 6 5 6 5 7 7 6 8 5 2 5 7 4 7 8 6 5 2 4 7 1 8 8 1 2 3 2 5 4 8 6 1 5 8 9 4 3 2 5 8 3 0 0 5 0 8 2 5 9 0 5 8 4 5 9 1 2 1 3 9 1 9 8 8 6 9 8 4 3 4 1 7 6 7 4 7 7 4 8 3 7 6 2 4 9 3 0 4 2 2 3 2 1 8 9 8 9 8 5 5 3 7 3 7 6 5 0 9 1 6 0 9 5 7 4 2 1 1 2 6 6 0 4 9 2 7 6 6 4 0 7 0 3 3 6 7 4 2 8 7 0 1 5 5 3 5 3 3 2 4 2 3 7 6 7 9 9 1 2 4 2 4 4 4 6 8 9 4 3 8 0 7 9 8 4 5 3 6 2 7 4 4 6 0 8 1 9 3 9 3 9 6 5 7 3 7 0 5 2 3 8 2 5 1 7 7 5 2 3 0 9 8 0 2 6 5 9 0 7 9 1 5 4 4 0 6 6 5 2 3 7 4 1 5 6 9 8 5 0 8 5 0 1 4 7 2 1 9 0 9 6 4 9 0 0 1 3 1 3 0 6 7 9 0 9 9 9 2 3 1 5 4 0 9 2 4 1 6 5 4 2 9 4 8 2 6 8 3 1 0 5 5 1 3 1 6 0 1 0 8 2 2 2 4 6 1 5 4 3 2 0 5 2 6 1 9 7 3 0 3 0 1 6 2 1 7 7 0 9 8 2 1 6 8 5 6 2 4 7 1 7 6 3 5 2 2 0 7 3 9 7 9 5 8 2 3 9 2 4 1 2 3 1 8 7 1 8 2 1 5 6 9 3 2 6 6 1 1 8 2 1 9 8 1 8 8 4 5 8 6 3 6 3 2 6 7 5 8 2 3 4 5 4 7 9 8 1 3 9 7 6 8 6 3 5 2 0 7 8 2 1 7 8 5 5 0 1 7 3 7 0 9 9 2 5 5 6 7 8 1 3 0 7 0 6 4 9 0 9 6 8 4 1 4 7 1 6 8 4 3 7 1 3 4 7 7 6 9 8 9 9 2 4 0 0 9 1 9 9 6 7 9 4 5 1 7 5 1 0 4 9 4 6 3 9 0 2 0 7 1 3 2 5 0 7 8 7 3 4 8 3 0 9 1 3 2 9 8 0 9 2 9 1 9 3 0 0 4 7 8 9 2 2 0 3 9 9 1 5 0 4 9 9 8 1 4 3 6 7 7 5 0 6 9 7 6 4 6 3 1 2 9 4 5 5 5 2 4 4 4 2 7 7 7 3 6 5 6 7 7 5 9 7 6 0 3 3 8 7 4 1 5 1 6 5 1 8 2 5 4 7 7 1 7 2 9 7 7 3 5 5 3 6 5 2 7 2 1 4 0 8 1 6 7 4 4 6 1 2 7 7 6 5 5 5 3 4 1 0 8 7 3 9 9 4 8 0 2 8 5 4 1 1 8 1 6 0 3 9 5 8 2 9 5 5 3 4 0 3 9 2 6 2 2 4 2 8 1 8 6 5 4 4 3 2 9 0 5 9 7 9 1 0 4 4 7 5 2 7 0 3 1 8 8 3 0 5 4 2 2 0 9 3 4 5 0 1 6 3 2 3 1 7 5 5 6 1 2 4 9 9 2 1 4 8 3 5 1 2 7 9 4 1 2 0 3 3 7 4 2 9 9 6 0 9 3 7 7 0 8 2 2 8 5 3 1 9 8 1 5 9 6 7 8 7 6 7 8 6 6 4 9 4 8 4 4 3 9 7 5 3 1 0 1 0 6 9 6 7 2 0 3 1 3 9 7 4 2 8 5 9 9 0 2 0 7 2 5 1 1 7 5 0 6 9 5 2 1 0 9 5 8 6 8 1 1 7 5 3 8 7 7 3 6 5 4 7 3 7 3 8 4 3 7 4 1 3 6 8 1 9 4 1 8 9 0 3 7 7 7 6 0 2 6 8 4 2 4 3 2 4 7 6 1 7 5 5 9 5 0 3 9 4 9 4 3 1 8 3 7 5 8 7 8 8 6 6 8 0 2 8 7 3 5 3 7 7 0 9 2 0 3 4 4 9 1 5 5 2 8 7 0 8 8 1 1 1 9 8 2 5 1 8 1 8 3 7 1 8 5 1 0 2 9 6 6 1 8 4 0 0 9 0 1 8 7 0 2 2 8 8 8 8 2 7 0 7 5 6 4 5 6 3 6 2 4 5 0 6 9 3 4 1 7 9 6 8 8 7 5 0 9 7 4 8 5 7 5 1 1 9 2 6 1 0 1 2 4 3 8 0 7 6 4 1 4 7 8 4 1 1 3 2 9 0 8 5 5 8 7 5 6 8 6 2 1 7 7 4 5 2 7 0 6 8 2 7 3 0 6 1 2 6 4 5 5 4 9 6 6 4 1 1 6 8 6 3 0 8 9 4 6 6 6 2 2 4 7 8 7 4 3 2 4 4 4 3 6 6 5 3 4 3 3 1 7 8 4 8 1 8 2 6 4 8 3 0 8 3 0 4 8
Here's a rigorous solution to go along with Darryl Stein's nice observations.
Use the Binomial Theorem to write x = ( 1 + 2 ) 3 0 0 0 = i = 0 ∑ 3 0 0 0 ( i 3 0 0 0 ) 2 i Notice that the even terms in this sum are integers. Notice that the binomial expansion of the "conjugate" expression, y = ( 1 − 2 ) 3 0 0 0 is the same thing, but with the odd terms negated: y = ( 1 − 2 ) 3 0 0 0 = i = 0 ∑ 3 0 0 0 ( i 3 0 0 0 ) ( − 2 ) i Adding these formulae, the odd terms cancel, and we see that x + y is a (fairly large) integer! x + y = 2 i = 0 ∑ 1 5 0 0 ( 2 i 3 0 0 0 ) 2 i Put another way, x and y are additive inverses (mod 1). Interestingly, x and y are also multiplicative inverses (over the reals), since ( 1 + 2 ) ( 1 − 2 ) = 1 − 2 = − 1 , and − 1 3 0 0 0 = 1 ; however, we will not need this fact.
Next we will show that 0 < y < 1 0 − 1 0 0 0 and hence y 's 1000'th digit after the decimal place is 0. Equivalently, 0 < ( 2 − 1 ) 3 < 1 0 1 which follows because 2 < 7 1 0 and ( 7 3 ) 3 = 3 4 3 2 7 < 1 0 1 .
Finally, since x and y are additive inverses (mod 1), and y has a 0 in its 1000'th decimal digit, and some nonzeros in later decimal digits, x must have a 9 in its 1000'th decimal digit. Here we also used the fact that both x and y are positive.