Powers of -4.

Terms

a(0) =1a(1) =-4a(2) =16a(3) =-64a(4) =256a(5) =-1024a(6) =4096a(7) =-16384a(8) =65536a(9) =-262144a(10) =1048576a(11) =-4194304a(12) =16777216a(13) =-67108864a(14) =268435456a(15) =-1073741824a(16) =4294967296a(17) =-17179869184a(18) =68719476736a(19) =-274877906944