[1] Agarwal, R. P., Berezansky, L., Braverman, E., Domoshnitsky, A.: 
Nonoscillation Theory of Functional Differential Equations with Applications. Springer, New York (2012). 
MR 2908263 | 
Zbl 1253.34002 
[4] Azbelev, N., Maksimov, V., Rakhmatullina, L.: 
Introduction to the Theory of Linear Functional Differential Equations. Advanced Series in Mathematical Science and Engineering 3 World Federation Publishers Company, Atlanta (1995). 
MR 1422013 | 
Zbl 0867.34051 
[7] Cabada, A.: 
An overview of the lower and upper solutions method with nonlinear boundary value conditions. Bound. Value Probl. 2011 (2011), Article ID 893753, 18 pages. 
MR 2719294 | 
Zbl 1230.34001 
[8] Domoshnitsky, A.: 
Maximum principles and nonoscillation intervals for first order Volterra functional differential equations. Dyn. Contin. Discrete Impuls. Syst., Ser. A, Math. Anal. 15 (2008), 769-814. 
MR 2469306 
[10] Graef, J. R., Henderson, J., Yang, B.: 
Existence and nonexistence of positive solutions of an $n$-th order nonlocal boundary value problem. Dynamic Systems and Applications 5 G. S. Ladde et al. Atlanta, GA (2008), 86-191. 
MR 2468138 | 
Zbl 1203.34026 
[11] Graef, J. R., Yang, B.: 
Positive solutions of a third order nonlocal boundary value problem. Discrete Contin. Dyn. Syst. Ser. S 1 (2008), 89-97. 
MR 2375585 | 
Zbl 1153.34014 
[12] Guidotti, P., Merino, S.: 
Gradual loss of positivity and hidden invariant cones in a scalar heat equation. Differ. Integral Equ. 13 (2000), 1551-1568. 
MR 1787081 | 
Zbl 0983.35013 
[13] Hakl, R., Lomtatidze, A., Šremr, J.: 
Some Boundary Value Problems for First Order Scalar Functional Differential Equations. Folia, Mathematica 10 Masaryk University, Brno (2002). 
MR 1909595 | 
Zbl 1048.34004 
[14] Infante, G.: 
Positive solutions of some nonlinear BVPs involving singularities and integral BCs. Discrete Contin. Dyn. Syst., Ser. S 1 (2008), 99-106. 
DOI 10.3934/dcdss.2008.1.99 | 
MR 2375586 
[18] Kiguradze, I., Půža, B.: 
Boundary Value Problems for Systems of Linear Functional Differential Equations. Folia, Mathematica 12 Masaryk University, Brno (2003). 
MR 2001509 | 
Zbl 1161.34300 
[19] Krasnosel'skij, M. A., Vainikko, G. M., Zabreiko, P. P., Rutitskij, Ya. B., Stetsenko, V. Ya.: 
Approximate Solution of Operator Equations. Russian Nauka, Moskva (1969). 
MR 0259635 
[22] Sommerfeld, A.: Ein Beitrag zur hydrodynamischen Erklärung der turbulenten Flüssig-keitsbewegungen. German Atti del IV Congresso Internazionale dei Matematici 3 Roma (1909), 116-124.
[23] Tchaplygin, S.: New Method of Approximate Integration of Differential Equations. GTTI, Moskva (1932), Russion.
[24] Webb, J. R. L.: 
Multiple positive solutions of some nonlinear heat flow problems. Discrete Contin. Dyn. Syst., suppl. (2005), 895-903. 
MR 2192752 | 
Zbl 1161.34007