2ξ1,ξ2]−∞,ξ1[]ξ1,ξ2[]ξ2,+∞[|I| =3| Tôi| -1=2 nút thắt).
Đối với splines khối (phổ biến)
Không có ràng buộc thường xuyên, chúng ta có phương trình:4 | Tôi| =12
1 ( ξ 1 ≤ X < ξ 2 ) ; 1 ( ξ 1 ≤ X < ξ 2 ) X ; ξ
1 (X< ξ1) ; 1 ( X < ξ1) X ; 1 ( X < ξ1) X2 ; 1 ( X < ξ1) X3 ;
1 ( ξ1≤ X< ξ2) ; 1 ( ξ 1≤ X< ξ2) X ; 1 ( ξ 1≤ X< ξ2) X2 ; 1 ( ξ 1≤ X< ξ2) X3 ;
1 ( ξ2≤ X) ; 1 ( ξ 2≤ X) X ; 1 ( ξ 2≤ X) X2 ; 1 ( ξ 2≤ X) X3.
Crr = 2(r+1)×(|I|−1)=3×(|I|−1)=6 constraints on the linear coefficients.
We end up with 12−6=6 degree of freedom.
For natural cubic splines
"A natural cubic splines adds additional constraints, namely that function is linear beyond the boundary knots."
Without regularity constraints, we have 4|I|−4=12−4 equations (we have removed 4 equations, 2 each in both boundary regions because they involve quadratic and cubic polynomials):
1(X<ξ1) ; 1(X<ξ1)X ;
1(ξ1≤X<ξ2) ; 1(ξ1≤X<ξ2)X ; 1(ξ1≤X<ξ2)X2 ; 1(ξ1≤X<ξ2)X3 ;
1(ξ2≤X) ; 1(ξ2≤X)X.
The constraints are the same as before, so we still need to add 3×(|I|−1)=6 constraints on the linear coefficients.
We end up with 8−6=2 degree of freedom.