Number of connected graphs with one cycle of length 4.

Terms

a(0) =1a(1) =1a(2) =4a(3) =9a(4) =28a(5) =71a(6) =202a(7) =542a(8) =1507a(9) =4114a(10) =11381a(11) =31349a(12) =86845a(13) =240567a(14) =668553a(15) =1860361a(16) =5188767a(17) =14495502a(18) =40572216a(19) =113743293a(20) =319405695a(21) =898288484a(22) =2530058013a(23) =7135848125a(24) =20152898513a(25) =56986883801