Sum_{i=0..n} Sum_{j=0..i} a(j) * a(i-j) = (-7)^n.

Terms

a(0) =1a(1) =-4a(2) =20a(3) =-116a(4) =708a(5) =-4452a(6) =28532a(7) =-185300a(8) =1215268a(9) =-8030404a(10) =53381844a(11) =-356577588a(12) =2391430020a(13) =-16092704292a(14) =108605848116a(15) =-734783381652