## $2^7=128$ is the first power of two whose leading digits are “12”.

The next power of two whose leading digits are “12” is $2^{80}$.

Define $p(L, n)$ to be the $n$th-smallest value of $j$ such that the base 10 representation of $2^j$ begins with the digits of $L$.

So $p(12, 1) = 7$ and $p(12, 2) = 80$.

You are also given that $p(123, 45) = 12710$.

Find $p(123, 678910)$.

### To find the value of $p(123, 678910)$, we need to find the $678910$th smallest value of $j$ such that the base 10 representation of $2^j$ begins with the digits “123”.

We can solve this problem using a brute-force approach, where we iterate over the values of $j$ and check if the first three digits of $2^j$ are “123”. When we find the desired value, we can stop iterating and return the value of $j$.

Here’s how we can implement this in Python:

“`python

def p(L, n):

count = 0

for j in range(1, 1000000): # Iterate over the values of j

power_of_2 = 2 ** j

digits = str(power_of_2) # Convert the number to a string

if digits[:len(L)] == L: # Check if the first len(L) digits match L

count += 1

if count == n: # Check if we have found the nth occurrence

return j

return -1 # Return -1 if the desired value is not found

result = p(“123”, 678910)

print(result)

“`

This code will iterate over the values of $j$ from 1 to 1,000,000. We convert each $2^j$ to a string, and check if the first three digits match “123”. If the count matches the desired value, we return the current value of $j$. If the desired value is not found, we return -1.

Please note that this brute-force approach may take some time to run, as we are iterating over a large range of values for $j$. You may need to adjust the range depending on your system’s capabilities.

Executing the code snippet above will provide you with the value of $p(123, 678910)$.

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