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Natural        The Second
Spline         derivative of the
               spline is zero at the
               endpoints.

Parabolic      The spline reduces
Runout         to a parabolic curve
Spline         on the first and last
               intervals.

Cubic          The spline is a
Runout         single cubic curve
Spline         on the first two and
               last two intervals.

The Natural Spline

The two simplest mathematical conditions we can impose are

These conditions together with 15 result in an linear system for , , …, , which can be written in matrix form as

For numerical calculations it is more convenient to eliminate and from this system and write

                                                                                                                                 (16)

together with

                                                                                                                                 (17)

                                                                                                                                 (18)

Thus, the                 linear system can be solved for the  coefficients , , …,            , and and are

determined by 17 and 18.

Physically, the natural spline results when the ends of a drafting spline extend freely beyond the interpolating points without

constraint. The end portions of the spline outside the interpolating points will fall on straight line paths, causing  to vanish at
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