### toam's blog

By toam, history, 6 weeks ago,

I sometimes make problems and post them in Japanese local contest yukicoder. I want many people to know math problems I made, so I introduce some of them. Let's try!!

## LCMST

#### Problem Statement

Consider a complete graph $G$ on $N$ vertices. The weight of edge between vertex $i$ and $j$ is least common multiple of $A_i$ and $A_j$. Find the total weight of the edges contained in a minimum spanning tree of $G$.

#### Constraints

• $2\leq N \leq 10^6$
• $1\leq A_i\leq 10^5$
• Time Limit is 4000ms

#### Sample

Consider the case where $N=3$ and $A=(2,3,4)$. The answer is $10$. This is optimal to draw edges between $(1,2),(1,3)$. These have weights $\mathrm{lcm}(A_1,A_2)=\mathrm{lcm}(2,3)=6, \mathrm{lcm}(A_1,A_3)=\mathrm{lcm}(2,4)=4$, respectively.

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## Simple Math ?

#### Problem Statement

You are given integer $a,N$. Find $\displaystyle \sum_{i=1}^N \lfloor \sqrt{ai}\rfloor\ \mathrm{mod}\ 10^9+7$. You have $T$ test cases to solve.

#### Constraints

• $1\leq T \leq 10$
• $1\leq a \leq 10^6$
• $1\leq N\leq 10^{12}$
• Time Limit is 2000ms

#### Sample

Consider the case where $a=2$ and $N=5$. The answer is $\lfloor \sqrt{2} \rfloor+\lfloor \sqrt{4} \rfloor+\lfloor \sqrt{6} \rfloor+\lfloor \sqrt{8} \rfloor+\lfloor \sqrt{10} \rfloor=1+2+2+2+3=10$.

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## Simple Math !

#### Problem Statement

You are given integers $P,Q,K$. Find the $K$-th smallest positive integer $N$ that satisfies the following conditions:

• There exists a pair of nonnegative integers $(x,y)$ such that $N=Px+Qy$

You have $T$ test cases to solve.

#### Constraints

• $1\leq T \leq 10^4$
• $1\leq P,Q,K \leq 10^9$
• Time Limit is 2000ms

#### Sample

Consider the case where $P=3,Q=4,K=4$. The answer is $7$.

• When $N=1$, there is no pair $(x,y)$ of nonnegative integers satisfying $1=3x+4y$.
• When $N=2$, there is no pair $(x,y)$ of nonnegative integers satisfying $2=3x+4y$.
• When $N=3$, $(x,y)=(1,0)$ satisfies $3=3x+4y$.
• When $N=4$, $(x,y)=(0,1)$ satisfies $4=3x+4y$.
• When $N=5$, there is no pair $(x,y)$ of nonnegative integers satisfying $5=3x+4y$.
• When $N=6$, $(x,y)=(2,0)$ satisfies $6=3x+4y$.
• When $N=7$, $(x,y)=(1,1)$ satisfies $7=3x+4y$.

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