打素数表？

刚刚水了道 Yandex 2013资格赛 的题目：

Prime problem

Time limit	3 seconds
Memory limit	256Mb
Input	stdin
Output	stdout

Legend

For a given integer n ≤ 4 ⋅ 10⁴, find all positive integers k ≤ n such that k²=p₁ ⋅ p₂+1, where p₁ and p₂are primes.

Input format

The first line of input contains one integer n (4 ≤ n ≤ 4 ⋅ 10⁴).

Output format

Print all appropriate values of k in arbitrary order. Separate consecutive values by spaces and/or newlines.

Sample 1

Input	Output
4	4

Sample 2

Input	Output
10	4 6

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题意： k²=p₁ ⋅ p₂+1，输出小于 n 的所有 k 值。

很简单的一道题，打表让我打坏了，只能搜模板了。搜到一个不错的模板。

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#include<stdio.h>
#include<math.h>
#include<string.h>

#define MAX_NUMBER 40010
int prime_number = 0;
int prime[MAX_NUMBER];
int ans[MAX_NUMBER];

void getPrime() {
    prime[++prime_number] = 2;
    for (int i = 3; i <= MAX_NUMBER; i++) {
        int j;
        for (j = 1; prime[j] * prime[j] <= i; j++) {
            if (i % prime[j] == 0) {
                break;
            }
        }
        if (prime[j] * prime[j] > i) {
            prime[++prime_number] = i;
        }
    }
}

int main(){
    int n;
    int cnt = 0;
    getPrime();
    for(int i=1; i<prime_number; i++){
        if(prime[i]+2==prime[i+1])
            ans[cnt++] = prime[i]+1;
    }
    while(scanf("%d", &n) != EOF){
        for(int i=0; i<cnt; i++){
            if(ans[i] <= n)
                printf("%d\n", ans[i]);
            else break;
        }
    }
    return 0;
}