a(n) is the number of convex pentagons in an n-triangular net.

Terms

a(0) =0a(1) =0a(2) =3a(3) =21a(4) =78a(5) =216a(6) =498a(7) =1014a(8) =1884a(9) =3264a(10) =5349a(11) =8379a(12) =12642a(13) =18480a(14) =26292a(15) =36540a(16) =49752a(17) =66528a(18) =87543a(19) =113553a(20) =145398a(21) =184008a(22) =230406a(23) =285714a(24) =351156a(25) =428064a(26) =517881a(27) =622167a(28) =742602a(29) =880992