On arithmetic progressions in symmetric sets in finite field model

We consider two problems regarding arithmetic progressions in symmetric sets in the finite field (product space) model. First, we show that a symmetric set S subset of Z(q)(n) containing vertical bar S vertical bar = mu . q(n) elements must contain at least delta(q, mu) . q(n) . 2(n) arithmetic progressions x, x+d, . . . , x+(q - 1).d such that the difference d is restricted to lie in {0, 1}(n). Second, we show that for prime p a symmetric set S subset of F-p(n) with vertical bar S vertical bar = mu . p(n) elements contains at least mu(C(p)) . p(2n) arithmetic progressions of length p. This establishes that the qualitative behavior of longer arithmetic progressions in symmetric sets is the same as for progressions of length three.