Problem:
Prove that there doesn't exist any prime such that every power of is a palindrome (palindrome is a number that is read the same from the left as it is from the right; in particular, number that ends in one or more zeros cannot be a palindrome).
Solutions — 2
Solution 1
Solution:
Note that by criterion for divisibility by and the definition of a palindrome we have that every palindrome that has even number of digits is divisible by .
Since is not a palindrome and since cannot divide for any prime other than we are now left to prove that no prime whose all powers have odd number of digits exists.
Assume the contrary. It means that the difference between the numbers of digits of and is even number. We will prove that for every natural , the difference is the same even number.
If we assume not, that means that the difference for some has at least digits more than the difference for some . We will prove that this is impossible.
Let , and , where
This implies that
which further implies that multiplying these powers of by can increase their number of digits by either or .
This is a contradiction. Call the difference between numbers of digits of consecutive powers . Number clearly cannot be equal to for because is divisible by two primes, but for , we would have that is a prime which is not true.
Case 1. . Let , for some real number greater than . (1)
From the definition of we also see that is smaller than . (2)
From (1) we see that powering gives us arbitrarily large numbers and from (2) we conclude that there is some natural power of , call it , greater than , such that
It is clear that has exactly digits more than has, which is an odd number, but sum of even numbers is even.
Case 2. . Let , for some real number greater than . (1) From the definition of we also see that is smaller than . (2)
From (1) we see that powering gives us arbitrarily large numbers and from (2) we conclude that there is some natural power of , call it , greater than , such that
It is clear that has exactly digits more than has, which is an odd number, but sum of even numbers is even.
We have now arrived at the desired contradiction for both cases and have thus finished the proof.
Solution 2
Solution:
Note that the sequence is periodic . Let the period be . Also, let .
Since all powers of are palindromes, all powers of are as well. Since is constant (mod ), the leftmost digit of each power of is equal to some .
We will prove that the difference between numbers of digits of and is equal to some for every natural number .
This is true due to size reasons. Namely, to add exactly digits, and yet to have the same leftmost digit, we need to multiply the number by at least (if then it's ) and by at most (values depend on the leftmost digit, it can easily be seen that leftmost digit being yields the extremal values). Notice that
Since this inequality has clearly shown that the interval of multipliers which add exactly digits and leave the leftmost digit the same is disjunct from the same kind of interval for digits, which implies that no number can belong to both intervals, we have successfully proven the claim.
Clearly, cannot be equal to for because a palindrome cannot be divisible by , but for we again cannot have the equality because is not a natural power of a prime.
Case 1. . Let , where is a real number, . (1) Here, is less than because if it was not, multiplying by would add at least digits, which is impossible.
From (1) we see that powering gives us arbitrarily large numbers and that there is some natural power of , call it , greater than , such that
Pick smallest such .
Now we easily see that the difference between numbers of digits of numbers and is exactly .
Case 2. . Let , where is a real number, . (2) Here, is less than because if it was not, multiplying by would add at most digits, which is impossible.
From (2) we see that powering gives us arbitrarily large numbers and that there is some natural power of , call it , greater than , such that
Pick smallest such .
Now we easily see that the difference between numbers of digits of numbers and is exactly .
We have arrived at the desired contradiction for both cases and have thus finished the proof.