Let and .
Define the operation on by (note the reverse order).
What is the value of ?
-1
Step-by-Step Solution
Key idea: This tests the definition of composition order and the multiplicative property of the sign function.
Step 1: Understand the operation.
. This means we apply first, then .
Step 2: Use the property of the sign function.
.
We do not need to compute the resulting permutation explicitly if we can find the signs of and individually.
Step 3: Find .
in one-line notation.
Inversions in :
- Pairs from : . (2 inversions)
- Pairs involving 5, 4: . (1 inversion)
- Check cross terms: ; ; . No inversions.
Total inversions for .
.
Alternatively, look at cycles: .
Sign = .
Step 4: Find .
in one-line notation.
Inversions in :
- is an inversion.
- is an inversion.
Total inversions for .
.
Alternatively, cycles: .
Sign = .
Step 5: Calculate .
.
Answer: -1