MathDB

There are 2 ‘i’ such that a_i<a_(i+1)

Source: 2013 Thailand October Camp Combinatorics Exam p2

March 7, 2022

combinatorics

Problem Statement

Find the number of permutations

(a_1, a_2, . . . , a_{2013})

(1, 2, \dots , 2013)

such that there are exactly two indices

i \in \{1, 2, \dots , 2012\}

where

a_i < a_{i+1}