HDU - 6760 Math is Simple (差分,莫比乌斯函数)
HDU - 6760 Math is Simple (差分,莫比乌斯函数)
题目链接:
http://acm.hdu.edu.cn/showproblem.php?pid=6760
Problem Description
Here is a simple math. You are given an integer n. Your task is to calculate the following:
思路:
我们尝试递推求\(F(n)\),
思考发现,从\(F(n-1)\)转移到\(F(n)\)需要曾增加 \(a+b=n\)的方案,减去\(a+b=n-1\)的方案。
即:
注意:上式中\(1\le a< b\leq n,1\le a< b\leq n-1\),
其实意义上严格的写法应该是\(1\le a< b< n\),但因为\(b=n\)时,\(\mathit a\)无取值,所以是等价的。
观察到右式中只有\(n,n-1\)之差,所以我们可以设:
又因为:
所以:
所以:
所以:
加入可得:
因此问题转化为求\(g(n)\)
莫比乌斯函数有性质:
我们将\(g(n)\)改写为枚举\(\mathit d\) 的形式,
因为:d是n的因子,又是a的因子 。a是\([1,n]\)中d的倍数,即: \(\frac{1}{a}=\frac{1}{d*i}\)
将右侧的\(\mathit d\) 提出来,
而\(S(x)\)我们可以通过\(O(n)\)预处理出来,
又因为内存限制,开\(1e8\) 大小的int数组,大概消耗$ 350Mb$,所以只能开一个\(1e8\)的数组,
\(\mu(n)\)我们就只能\(O(\sqrt{n}+2^{pd(n)})\)时间复杂度求解,
其中\(pd(n)\)是n的素因子种类数,在\(n\leq 10^8\),\(pd(n)\leq 8\)。即前8个质数相乘大于\(1e8\)。
因为一个数\(\mathit x\)的某个质因子次幂大于1时,\(\mu(x)=0\),所以我们只需要\(O(\sqrt n)\)得到\(\mathit n\) 的所有质因子,然后\(O(2^{pd(n)})\)二进制枚举所有\(\mu(d)\not=0\)的\(\mathit d\) 即可,即一个因子取或不取。
所以本题的时间复杂度为:
\(O(10^8+T*(\sqrt{n}+2^{pd(n)}))\) 大概在\(2e8\)范围内,可以接受。
代码:
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <bits/stdc++.h>
#define ALL(x) (x).begin(), (x).end()
#define sz(a) int(a.size())
#define rep(i,x,n) for(int i=x;i<n;i++)
#define repd(i,x,n) for(int i=x;i<=n;i++)
#define pii pair<int,int>
#define pll pair<long long ,long long>
#define gbtb ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
#define MS0(X) memset((X), 0, sizeof((X)))
#define MSC0(X) memset((X), '\0', sizeof((X)))
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define eps 1e-6
#define chu(x) if(DEBUG_Switch) cout<<"["<<#x<<" "<<(x)<<"]"<<endl
#define du3(a,b,c) scanf("%d %d %d",&(a),&(b),&(c))
#define du2(a,b) scanf("%d %d",&(a),&(b))
#define du1(a) scanf("%d",&(a));
using namespace std;
typedef long long ll;
ll gcd(ll a, ll b) {return b ? gcd(b, a % b) : a;}
ll lcm(ll a, ll b) {return a / gcd(a, b) * b;}
ll powmod(ll a, ll b, ll MOD) { if (a == 0ll) {return 0ll;} a %= MOD; ll ans = 1; while (b) {if (b & 1) {ans = ans * a % MOD;} a = a * a % MOD; b >>= 1;} return ans;}
ll poww(ll a, ll b) { if (a == 0ll) {return 0ll;} ll ans = 1; while (b) {if (b & 1) {ans = ans * a ;} a = a * a ; b >>= 1;} return ans;}
void Pv(const vector<int> &V) {int Len = sz(V); for (int i = 0; i < Len; ++i) {printf("%d", V[i] ); if (i != Len - 1) {printf(" ");} else {printf("\n");}}}
void Pvl(const vector<ll> &V) {int Len = sz(V); for (int i = 0; i < Len; ++i) {printf("%lld", V[i] ); if (i != Len - 1) {printf(" ");} else {printf("\n");}}}
inline long long readll() {long long tmp = 0, fh = 1; char c = getchar(); while (c < '0' || c > '9') {if (c == '-') fh = -1; c = getchar();} while (c >= '0' && c <= '9') tmp = tmp * 10 + c - 48, c = getchar(); return tmp * fh;}
inline int readint() {int tmp = 0, fh = 1; char c = getchar(); while (c < '0' || c > '9') {if (c == '-') fh = -1; c = getchar();} while (c >= '0' && c <= '9') tmp = tmp * 10 + c - 48, c = getchar(); return tmp * fh;}
void pvarr_int(int *arr, int n, int strat = 1) {if (strat == 0) {n--;} repd(i, strat, n) {printf("%d%c", arr[i], i == n ? '\n' : ' ');}}
void pvarr_LL(ll *arr, int n, int strat = 1) {if (strat == 0) {n--;} repd(i, strat, n) {printf("%lld%c", arr[i], i == n ? '\n' : ' ');}}
const int maxn = 100000010;
const int inf = 0x3f3f3f3f;
/*** TEMPLATE CODE * * STARTS HERE ***/
#define DEBUG_Switch 0
const int mod = 998244353;
int inv[maxn];
// inv[i]= (1/i) % mod
int ans;
int p[30];
int cnt;
int n;
void dfs(int pos, int now, int u)
{
if (pos > cnt)
{
ans = (ans + 1ll * u * ((inv[now] - inv[now - 1] + mod) % mod) % mod * inv[n / now ] % mod) % mod;
return ;
}
dfs(pos + 1, now, u);
dfs(pos + 1, now * p[pos], mod - u);
}
int main()
{
#if DEBUG_Switch
freopen("C:\\code\\input.txt", "r", stdin);
freopen("C:\\code\\output.txt", "w", stdout);
#endif
inv[1] = 1;
for (int i = 2; i < maxn; i++)
inv[i] = 1ll * (mod - mod / i) * inv[mod % i] % mod;
for (int i = 2; i < maxn; i++)
inv[i] = ( inv[i] + inv[i - 1]) % mod;
int t;
t = readint();
while (t--)
{
ans = 0ll; cnt = 0;
n = readint();
if (n <= 2)
{
printf("%d\n", (inv[2] - 1 + mod) % mod);
continue;
}
int temp = n;
for (int i = 2; i * i <= temp; ++i)
{
if (temp % i == 0)
{
p[++cnt] = i;
while (temp % i == 0)
temp /= i;
}
}
if (temp > 1)
p[++cnt] = temp;
dfs(1, 1, 1);
ans = (1ll * ans * ((inv[n] - inv[n - 1] + mod) % mod)) % mod;
ans = (ans + ((inv[2] - 1 + mod) % mod)) % mod;
printf("%d\n", ans );
}
return 0;
}