Problem
COM-B2-M03-P010 No Consecutive Numbers
#10
★★★☆☆ Level 3 of 5
How many \(5\)-element subsets of \(\{1,\ldots,18\}\) contain no consecutive numbers?
If \(a_1<\cdots
The no-adjacency condition gives \(a_{i+1}\ge a_i+2\). Then \(b_i=a_i-i+1\) form an ordinary \(5\)-subset of \(\{1,\ldots,14\}\). Conversely, from \(b_1<\cdots
A strong but standard shifting bijection.