Supremum of a concave function
4 views (last 30 days)
Show older comments
I have a function I want to calculate its supremum. The function is below.
-0.25* (c+A^t-v)^T *(c+A^t-v)/v for all v>0
2 Comments
Image Analyst
on 1 Jan 2021
Edited: Image Analyst
on 1 Jan 2021
numPoints = 1000;
v = linspace(0.02, 1, 1000);
% Guesses:
c = 1 * ones(1, numPoints);
A = 2 * ones(1, numPoints);
T = 2 * ones(1, numPoints);
t = 3 * ones(1, numPoints);
% Compute function
y = -0.25 * (c+A.^t-v).^T .* (c+A.^t-v)./v %for all v>0
% Plot it.
plot(v, y, 'b.-', 'LineWidth', 2);
grid on;
What are c, A, t, and T?
John D'Errico
on 1 Jan 2021
Edited: John D'Errico
on 1 Jan 2021
What do you know about c, A, t, and T? T is most important, of course. For example, if T is not an integer, then things are, let me say, difficult? That is because noninteger powers of negative numbers will be complex, so that supremem will be a nasty thing.
Answers (1)
Walter Roberson
on 2 Jan 2021
This creates a list of supermum for the function, together with the conditions under which the supermum hold. The calculations would have been easier if we had been given more information about the symbols.
syms A t T v c;
f = -0.25* (c+A^t-v)^T *(c+A^t-v)/v;
df = diff(f,v);
sol = solve(df == 0,v,'returnconditions', true);
flavor = simplify(subs(diff(df,v),v,sol.v));
conditional_flavor = arrayfun(@(F,C) simplify(piecewise(C & F>0,symtrue,nan)), flavor, sol.conditions);
bs1 = [T == -1, T==0, T==1, T~=-1 & T~=0 & T~=1 & 1<real(T), T~=-1 & T~=0 & T~=1 & 1>real(T)];
bs2 = [c + A^t~=0, c + A^t==0];
branches = and(bs1, bs2(:));
for bidx = 1 : numel(branches)
assume(assumptions, 'clear')
assume(branches(bidx));
constrained_conditions(:,bidx) = simplify(conditional_flavor);
end
assume(assumptions, 'clear')
supermum= [];
for K = 1: size(constrained_conditions,1)
for bidx = find(~isnan(constrained_conditions(K,:)))
temp = arrayfun(@(C) simplify(piecewise(v == sol.v(K) & C, subs(f, v, sol.v(K)))), branches(bidx) & constrained_conditions(K,bidx));
supermum = [supermum; temp];
end
end
supermum
There is also a saddle point of f = 0 when v = c + A^t
0 Comments
See Also
Categories
Find more on Logical in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!