Fix positive integers $n,j,k$.How many integer sequences are there of the form $1\le a_1<a_2<\ldots<a_k\le n$,where $a_{i+1}-a_i\ge j$ for all $1\le i\le k-1$.
Problem
Source: India Postal Coaching 2014 Set 6 Problem 2
Tags: combinatorics unsolved, combinatorics