Number of tilings of a 21 X n rectangle using 3n heptominoes of shape I.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =1a(5) =1a(6) =1a(7) =53a(8) =141a(9) =271a(10) =449a(11) =681a(12) =973a(13) =1331a(14) =9391a(15) =26803a(16) =60953a(17) =122641a(18) =226921a(19) =394031a(20) =650413a(21) =2039288a(22) =5611209a(23) =13694084a(24) =30417897a(25) =62763334a(26) =122046063a(27) =225958222a(28) =532867801a(29) =1319078776