Study of fractional semipositone problems on ${\mathbb{R}}^N$

Let $s\in (0,1)$ and $N>2s$. In this paper, we consider the following class of nonlocal semipositone problems: \begin{align*} (-Δ)^s u= g(x)f_a(u) \text { in } $\mathbb{R}$^N, ; u > 0 $\text{ in }$ $\mathbb{R}$^N, \end{align*} where the weight $g\in L^1({\mathbb{R}}^N)\cap L^{\mathrm{\infty }}({\mathbb{R}}^N)$ is positive, $a>0$ is a parameter, and $f_a\in \mathcal{C}(\mathbb{R})$ is strictly negative on $(-\mathrm{\infty },0]$. For $f_a$ having subcritical growth and weaker Ambrosetti-Rabinowitz type nonlinearity, we prove that the above problem admits a mountain pass solution $u_a$, provided `$a$’ is near zero. To obtain the positivity of $u_a$, we establish a Brezis-Kato type uniform estimate of $(u_a)$ in $L^r({\mathbb{R}}^N)$ for every $r\in [\frac{2N}{N-2s},\mathrm{\infty }]$.