Lomtatidze, A., Vodstrčil, P. On sign constant solutions of certain boundary value problems for second-order functional differential equations. Applicable Analysis. 2005, 84(2), 197-209. ISSN 0003-6811. E-ISSN 1563-504X. Dostupné z: http://www.tandfonline.com/doi/full/10.1080/00036810410001724427