Thank you for the help so far!
I would like to assign +/i signs for the function and solution such as (+)(+)(-) and therefore (-).
It is working for the first evaluation, [Lambda]lowerλlower, but not for the second evaluation, [Lambda]upperλupper. I cannot see the difference - has it to do with "[Lambda]i"λi /. Solve" in both definitions?
(* Endogenous *)
Clear[\[Mu]Clear[μ, \[Xi]ξ, \[Beta]β, \[Gamma]γ, ri, si, rj, sj, \[Lambda]CriλCri, \[Lambda]CsiλCsi , \[Lambda]lowerλlower, \[Lambda]upperλupper, \[Lambda]lowereipλlowereip, \[Lambda]ip];λip];
(* Exogenous *)
Clear[ ci, yp, \[Lambda]iλi, \[Alpha]iαi, \[Epsilon]ipϵip, \[Eta]ipηip, eip];
ri = \[Epsilon]ip*\[Lambda]iϵip*λi + (1 - \[Eta]ipηip)*(1 - \[Lambda]iλi);
si = \[Eta]ip*ηip*(1 - \[Lambda]iλi) + (1 - \[Epsilon]ipϵip)*\[Lambda]i;*λi;
\[Lambda]CriλCri = (\[Epsilon]ip*\[Lambda]iϵip*λi)/(\[Epsilon]ip*\[Lambda]iϵip*λi + (1 - \[Eta]ipηip)*(1 - \[Lambda]iλi));
\[Lambda]CsiλCsi = (\[Eta]ip*ηip*(1 - \[Lambda]iλi))/(\[Eta]ip*ηip*(1 - \[Lambda]iλi) + (1 - \[Epsilon]ipϵip)*\[Lambda]i*λi);
\[Lambda]lowerλlower = \[Lambda]iλi /.
Solve[ri*(yp - eip - ci + \[Alpha]i*\[Lambda]Criαi*λCri) + si*(yp - eip + \[Alpha]i*αi*(1 - \[Lambda]CsiλCsi)) == yp + \[Alpha]i*αi*(1 - \[Lambda]iλi), {\[Lambda]iλi}];
\[Lambda]upperλupper = \[Lambda]iλi /.Solve[ri*(yp - eip - ci + \[Alpha]i*\[Lambda]Criαi*λCri) +
si*(yp - eip + \[Alpha]i*αi*(1 - \[Lambda]CsiλCsi)) == yp + \[Alpha]i*\[Lambda]iαi*λi - ci, {\[Lambda]iλi} ];
Simplify[D[\[Lambda]lowerSimplify[D[λlower, eip]]
Simplify[Sign[D[\[Lambda]lowerSimplify[Sign[D[λlower, eip]], \[Alpha]iαi > 0 && eip > 0 && ci > 0 && 0 < \[Epsilon]ipϵip < 1 && 0 < ci < 1 && 0 < \[Eta]ipηip < 1 && \[Alpha]iαi > ci && \[Alpha]iαi > ci + eip && \[Epsilon]ipϵip + \[Eta]iηi < 1]
Simplify[D[\[Lambda]upperSimplify[D[λupper, eip]]
Simplify[Sign[D[\[Lambda]upperSimplify[Sign[D[λupper, eip]], \[Alpha]iαi > 0 && eip > 0 && ci > 0 && 0 < \[Epsilon]ipϵip < 1 && 0 < ci < 1 && 0 < \[Eta]ipηip < 1 && \[Alpha]iαi > ci && \[Alpha]iαi > ci + eip && \[Epsilon]ipϵip + \[Eta]iηi < 1]