### sevlll's blog

By sevlll, history, 4 months ago,

Hi!

Today I was surfing Wikipedia and came across this article — Palindromic prime

This article says that the largest known palindromic prime is $10^{474500}$ + $999 * 10^{237249} + 1$.

Well it is easy to see that this number is palindrome, but... why is it prime?

I don't find any proof, and I am really curios — how to proof that this number is prime, when number is quite big?

• +79

By sevlll, history, 14 months ago,

DISCLAIMER: sorry for my poor english, hope you can understand this text :)

Hello!

Firstly: Codeforces is a beautiful platform, sure.

In my mind hacks became useless. Why? Ok, see:

Many easy problems (D1AB/D2ABCD) are "multitest" problems. It is really hard to hack them, because pretests are very strong. But what if I want to hack some hard problems? Hm, i tried to hack some hard problems, but in div2 rooms are very small, and i discovered that there were only 2-3 people who solved some hard problems.

Anyway, i saw that Retired_MiFaFaOvO in one of his screencasts was very annoyed about it too.

But if i will try to hack D2B for example, i would find roughly 10 people, and the chance of succes hack is very small, so it is useless to spend time on it.

So, it is really hard to hack someone, and we can see that the quantity of hacks is very small in some previous rounds!

-

What is my solution? Ok, lets make rooms bigger from 40 to 80.