Number of length-2n binary strings of the form xxyy.

Terms

a(0) =2a(1) =6a(2) =20a(3) =54a(4) =146a(5) =346a(6) =848a(7) =1922a(8) =4424a(9) =9810a(10) =21816a(11) =47598a(12) =103730a(13) =223498a(14) =480542a(15) =1025956a(16) =2184362a(17) =4629670a(18) =9786512a(19) =20618936a(20) =43340862a(21) =90870526a(22) =190141128a(23) =397049128a(24) =827675318