Abstract. In this paper, the existence of least energy solution and infinitely many solutions is proved for the equation (1 − ∆) α u = f (u) in R N where 0 < α < 1, N ≥ 2 and f (s) is a Berestycki-Lions type nonlinearity. The characterization of the least energy by the mountain pass value is also considered and the existence of optimal path is shown. Finally, exploiting these results, the existence of positive solution for the equation (1 − ∆) α u = f (x, u) in R N is established under suitable conditions on f (x, s).
IntroductionIn this paper, we are concerned with the existence of nontrivial solutions of (1) (where N ≥ 2 and 0 < α < 1. The fractional operator (1 − ∆) α u is defined byand H α (R N ) a fractional Sobolev space consisted by real valued functions, that is,Throughout this paper, we deal with a weak solution of (1), namely, a functionwhere a denotes the complex conjugate of a. The operator (1 − ∆) α is related to the pseudo-relativistic Schrödinger operator (m 2 − ∆) 1/2 − m (m > 0) and recently a lot of attentions are paid for equations involving them. Here we refer to [2-4, 12-17, 21, 23, 32, 34, 38] and references therein for more details and physical context of (1 − ∆) α . In these papers, the authors study the existence of nontrivial solution and infinitely many solutions for the equations with (m 2 − ∆) α and various nonlinearities. This paper is especially motivated by two papers [23] and [34]. In [23], the existence of positive solution of (1) is proved under the following conditions on f (x, s):(vi) There exist continuous functionsf (s) and a(x) such thatf satisfies (i)-(v) and 0On the other hand, in [34], the author obtains a nontrivial solution of (1) with f (x, s) = λb(x)|u| p−1 u + c(x)|u| q−1 u under different conditions on b(x), c(x), p, q where λ > 0 is a constant. Among other things, under 1 < p, q < 2 * α − 1 and some strict inequality for the mountain pass value (the infimum of the functional on the 2010 Mathematics Subject Classification. 35J60, 35S05.