In this paper, we consider positive supersolutions of the semilinear fourthorder problem
where Ω is a domain in \IR^{N} (bounded or not), f:D_{f} = [0,a_{f}) → [0,∞) (0 < a_{f} \leqslant +∞) is a nondecreasing continuous function with f(u) > 0 for u > 0 and ρ: Ω→ \IR is a positive function. Using a maximum principlebased argument, we give explicit estimates on positive supersolutions that can easily
be applied to obtain Liouvilletype results for positive supersolutions either in exterior domains, or in unbounded domains Ω with the property that sup_{x ∈ Ω}dist (x,∂Ω)=∞. In particular, we consider the above problem with f(u)=u^{p} (p > 0) and with different weights ρ(x)=x^{a}, e^{ax1} or x_{1}^{m} (m is an even integer).
Also, when f is
convex and ρ:Ω→ (0,∞) is smooth with ∆(√{ρ}) > 0, then under an extra condition between f and ρ we show that every positive supersolution u of this problem with u=0 on ∂Ω (Ω bounded) satisfies the inequality
−∆u ≥ √{2ρ(x)F(u)} for all x ∈ Ω,
where F(t):=∫_{0}^{t} (f(s)−f(0))ds.
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