Abstract. We compute the degree complexity of the family of birational maps considered in [2] for all exceptional cases. Some interesting properties of the family are also given.
IntroductionWe continue the work of [2] where we considered a family k F of birational maps of the plane determined by a choice of polynomial F (w) = a 0 + a 1 w + . . . + a n w n (the definition of the family k F will be recalled in Section 2). In [2] we determined the degree complexity δ(k F ) in the generic case. If δ(k b F ) is less than the generic value, then we say that F is exceptional. (The set of exceptional parameters is a nowhere dense algebraic subset.) This corresponds to cases of degree reduction which are especially interesting because they correspond to the maps that have special symmetries.As seen in [2], there is a fundamental difference between the cases where n, the degree of F, is even or odd.The complexity degree δ(k F ) in the case n is even is given byCase 2: If a 0 = 2 1+m for some integer m ≥ 0 then δ(k F ) is the largest real root of the polynomial x 2m+1 (x 2 − (n + 1)x − 1) + x 2 + n.The complexity degree δ(k F ) in the case n is odd is given by 0 , a 1 , . . . , a n ) = −(a n−j + a n−j+1 ) − [−a n n j + a n−1with n! = n(n − 1) . . . 2.1 the factorial of n. a 0 , a 1 , . . . , a n ) = 0 for all 0 ≤ j ≤ h.Case 1: h < n − 2, and a 0 = 2/(1 + m) for all integers m ≥ 0. Then δ(k F ) is the largest real root of the polynomial x 3 − nx 2 − (n + 1 − h)x − 1.Case 2: h < n − 2, and a 0 = 2/(1 + m) for some integer m ≥ 0. Then δ(k F ) is the largest real root of the polynomial x 2m+1 (x 3 − nx 2 − (n − h + 1)x − 1) + x 3 + x 2 + nx + n − h − 1.Case 3: h = n − 2, and a 0 = 2/(1 + m) for all integers m ≥ 0, and a 0 = n+1 2 + l 2(1+l) for all integers l ≥ 0. Then δ(k F ) is the largest real root of the polynomial x 3 − nx 2 − 2x − 1.Case 4: h = n − 2, and a 0 = 2/(1 + m) for some integer m ≥ 0, and a 0 = n+1 2 + l 2(1+l) for all integers l ≥ 0. Then δ(k F ) is the largest real root of the polynomial x 2m (x 3 − nx 2 − 2x − 1) + x 2 + x + n.Case 5: h = n − 2, and a 0 = 2/(1 + m) for all integers m ≥ 0, and a 0 = n+1 2 + l 2(1+l) for some integer l ≥ 0. Then δ(k F ) is the largest real root of the polynomial x 2l+2 (x 3 − nx 2 − 2x − 1) + nx 2 + x + 1.Case 6: h = n − 2, and a 0 = 2/(1 + m) for some integer m ≥ 0, and a 0 = n+1 2 + l 2(1+l) for some integer l ≥ 0. Then n = 3, a 0 = 2, and the map k F is exactly the family considered in Section 5 in [2]. Hence in this case δ(k F ) = 1.