Is the Ramsey model one with non negative investment? I have a very fundamental question. I am currently searching for an economic growth model with non negative investment. Can someone tell me if the Ramsey model is one with non negative investment and if so - why? Thank you very much
No it is not. The benchmark Ramsey model allows for positive and negative investment. Treating the case of non-negative (or &quot;irreversible&quot; investment) is considered an extension.
See Barro &amp; Sala-i-Martin (2004). Economic growth (2nd ed)

Appendix 2B for a short discussion and
Section 5.1.2 pp 242-247 for an Endogenous growth model with irreversible investment and Human capital.

For a more deep treatment, see
Arrow and Kurz(1970).  Optimal Growth with Irreversible Investment in a Ramsey Model, Econometrica, Vol. 38(2), 331-344 .