9552: ABC239 —— Ex - Dice Product 2
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Description
Snuke has a die (singular of dice) that shows integers from $1$ through $N$ with equal probability, and an integer $1$.
He repeats the following operation while his integer is less than or equal to $M$.
- He rolls the die. If the die shows an integer $x$, he multiplies his integer by $x$.
Find the expected value of the number of times he rolls the die until he stops, modulo $10^9+7$.
Definition of the expected value modulo $10^9+7$ We can prove that the desired expected value is always a rational number. Moreover, under the constraints of the problem, when the value is represented as an irreducible fraction $\frac{P}{Q}$, we can also prove that $Q \not\equiv 0 \pmod{10^9+7}$. Thus, an integer $R$ such that $R \times Q \equiv P \pmod{10^9+7}$ and $0 \leq R \lt 10^9+7$ is uniquely determined. Answer such $R$.
He repeats the following operation while his integer is less than or equal to $M$.
- He rolls the die. If the die shows an integer $x$, he multiplies his integer by $x$.
Find the expected value of the number of times he rolls the die until he stops, modulo $10^9+7$.
Definition of the expected value modulo $10^9+7$ We can prove that the desired expected value is always a rational number. Moreover, under the constraints of the problem, when the value is represented as an irreducible fraction $\frac{P}{Q}$, we can also prove that $Q \not\equiv 0 \pmod{10^9+7}$. Thus, an integer $R$ such that $R \times Q \equiv P \pmod{10^9+7}$ and $0 \leq R \lt 10^9+7$ is uniquely determined. Answer such $R$.
Input
Input is given from Standard Input in the following format:
```
$N$ $M$
```
```
$N$ $M$
```
Output
Print the answer.
Constraints
- $2 \leq N \leq 10^9$
- $1 \leq M \leq 10^9$
- $1 \leq M \leq 10^9$
Sample 1 Input
2 1
Sample 1 Output
2
The answer is the expected value of the number of rolls until it shows 2 for the first time. Thus, 2 should be printed.
Sample 2 Input
2 39
Sample 2 Output
12
The answer is the expected value of the number of rolls until it shows 2 six times. Thus, 12 should be printed.
Sample 3 Input
3 2
Sample 3 Output
250000004
The answer is $\frac{9}{4}$. We have $4×250000004≡9\ (\bmod\ {10^9+7+)$, so 250000004 should be printed.
2392 39239
984914531
Sample 4 Input
1000000000 1000000000
Sample 4 Output
776759630