Maths Olympiad Prep

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Number theory Difficulty 6.0 AIME, harder Prove it JBMO

Problem:
Prove that there doesn't exist any prime pp such that every power of pp 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 1111 and the definition of a palindrome we have that every palindrome that has even number of digits is divisible by 1111.
Since 115=16105111^{5} = 161051 is not a palindrome and since 1111 cannot divide pkp^{k} for any prime other than 1111 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 pmp^{m} and pm+1p^{m+1} is even number. We will prove that for every natural mm, the difference is the same even number.
If we assume not, that means that the difference for some m1m_{1} has at least 22 digits more than the difference for some m2m_{2}. We will prove that this is impossible.
Let pm1=10t1a1p^{m_{1}} = 10^{t_{1}} \cdot a_{1}, pm2=10t2a2p^{m_{2}} = 10^{t_{2}} \cdot a_{2} and p=10hzp = 10^{h} \cdot z, where
1<a1,a2,z<10 1 < a_{1}, a_{2}, z < 10
This implies that
1<a1z, a2z<100 1 < a_{1} \cdot z,\ a_{2} \cdot z < 100
which further implies that multiplying these powers of pp by pp can increase their number of digits by either hh or h+1h+1.
This is a contradiction. Call the difference between numbers of digits of consecutive powers dd. Number pp clearly cannot be equal to 10d10^{d} for d1d \geq 1 because 1010 is divisible by two primes, but for d=0d=0, we would have that 11 is a prime which is not true.

Case 1. p>10dp > 10^{d}. Let p=10dap = 10^{d} \cdot a, for some real number aa greater than 11. (1)
From the definition of dd we also see that aa is smaller than 1010. (2)
From (1) we see that powering aa gives us arbitrarily large numbers and from (2) we conclude that there is some natural power of aa, call it bb, greater than 11, such that
10<ab<100 10 < a^{b} < 100
It is clear that pbp^{b} has exactly (b1)d+1(b-1)d+1 digits more than pp has, which is an odd number, but sum of even numbers is even.

Case 2. p<10dp < 10^{d}. Let p=10dap = \frac{10^{d}}{a}, for some real number aa greater than 11. (1) From the definition of dd we also see that aa is smaller than 1010. (2)
From (1) we see that powering aa gives us arbitrarily large numbers and from (2) we conclude that there is some natural power of aa, call it bb, greater than 11, such that
10<ab<100 10 < a^{b} < 100
It is clear that pbp^{b} has exactly (b1)d1(b-1)d-1 digits more than pp 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 {pn}\{p^{n}\} is periodic (mod 10)(\bmod\ 10). Let the period be dd. Also, let pd=gp^{d} = g.
Since all powers of pp are palindromes, all powers of gg are as well. Since {gn}\{g^{n}\} is constant (mod 1010), the leftmost digit of each power of gg is equal to some ff.
We will prove that the difference between numbers of digits of gmg^{m} and gm+1g^{m+1} is equal to some rr for every natural number mm.
This is true due to size reasons. Namely, to add exactly kk digits, and yet to have the same leftmost digit, we need to multiply the number by at least 510k15 \cdot 10^{k-1} (if k=0k=0 then it's 11) and by at most 210k2 \cdot 10^{k} (values depend on the leftmost digit, it can easily be seen that leftmost digit being 11 yields the extremal values). Notice that
210k<510k+11 2 \cdot 10^{k} < 5 \cdot 10^{k+1-1}
Since this inequality has clearly shown that the interval of multipliers which add exactly kk digits and leave the leftmost digit the same is disjunct from the same kind of interval for k+1k+1 digits, which implies that no number can belong to both intervals, we have successfully proven the claim.
Clearly, gg cannot be equal to 10r10^{r} for r1r \geq 1 because a palindrome cannot be divisible by 1010, but for r=0r=0 we again cannot have the equality because 11 is not a natural power of a prime.

Case 1. g>10rg > 10^{r}. Let g=10rag = 10^{r} \cdot a, where aa is a real number, 10>a>110 > a > 1. (1) Here, aa is less than 1010 because if it was not, multiplying by gg would add at least r+1r+1 digits, which is impossible.
From (1) we see that powering aa gives us arbitrarily large numbers and that there is some natural power of aa, call it bb, greater than 11, such that
10<ab<100 10 < a^{b} < 100
Pick smallest such bb.
Now we easily see that the difference between numbers of digits of numbers gb1g^{b-1} and gbg^{b} is exactly r+1r+1.

Case 2. g<10rg < 10^{r}. Let g=10rag = \frac{10^{r}}{a}, where aa is a real number, 10>a>110 > a > 1. (2) Here, aa is less than 1010 because if it was not, multiplying by gg would add at most r1r-1 digits, which is impossible.
From (2) we see that powering aa gives us arbitrarily large numbers and that there is some natural power of aa, call it bb, greater than 11, such that
10<ab<100 10 < a^{b} < 100
Pick smallest such bb.
Now we easily see that the difference between numbers of digits of numbers gb1g^{b-1} and gbg^{b} is exactly r1r-1.

We have arrived at the desired contradiction for both cases and have thus finished the proof.

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