[1] L. D. Berkovitz, H. Pollard: A nonclassical variational problem arising from an optimal filter problem II. Arch. Ratl. Mech. Anal. 38 (1970), 161-172. DOI 10.1007/BF00251656 | MR 0270247

[2] R. С. Brown: Duality theory for $n$-th order differential operators under Stieltjes boundary conditions. S.I.A.M. J. Math. Anal., to appear. MR 0385224 | Zbl 0316.47027

[3] R. C. Brown: Duality theory for $n$-th order differential operators under Stieltjes boundary conditions, II: Nonsmooth coefficients and nonsingular measures. Ann. Mat. Рurа. Appl., to appear. MR 0422745 | Zbl 0316.47027

[4] R. C. Brown: Adjoint domains and generalized splines. Czech. Math. J., 25 (1975), 134-147. MR 0397243 | Zbl 0309.41014

[5] N. Dunford, J. T. Schwartz: Linear operators. Part 1, Interscience, New York, 1975.

[6] S. Goldberg: Unbounded linear operators. McGraw-Hill, New York, 1966. MR 0200692 | Zbl 0148.12501

[7] M. Golomb, J. Jerome: Linear ordinary differential equations with boundary conditions on arbitrary point sets. Trans. Amer. Math. Soc., 153 (1971), 235 - 264. DOI 10.1090/S0002-9947-1971-0269918-3 | MR 0269918 | Zbl 0238.34027

[8] A. M. Krall, R. С Brown: $n$-th order differential systems under Stieltjes boundary conditions. MRC Tech. Summ. Rept. #1581.

[9] D. G. Lunberger: Optimization by vector space methods. John Wiley, New York, 1969. MR 0238472

[10] L. Schumaker: Spline functions: Theory and applications. in press.