Number of corners in a 4-sided fractal.

Terms

a(0) =4a(1) =8a(2) =20a(3) =60a(4) =204a(5) =748a(6) =2860a(7) =11180a(8) =44204a(9) =175788a(10) =701100a(11) =2800300a(12) =11193004a(13) =44755628a(14) =178989740a(15) =715893420a(16) =2863442604a(17) =11453508268a(18) =45813508780a(19) =183252986540a(20) =733009849004