a(n) = (A001359(n+1)^2 - 1)/24, where A001359 = lesser of twin primes; or: pentagonal numbers (A000326) whose indices are twin ranks (A002822).

A308344

a(n) = (A001359(n+1)^2 - 1)/24, where A001359 = lesser of twin primes; or: pentagonal numbers (A000326) whose indices are twin ranks (A002822).

Terms

    a(0) =1a(1) =5a(2) =12a(3) =35a(4) =70a(5) =145a(6) =210a(7) =425a(8) =477a(9) =782a(10) =925a(11) =1335a(12) =1520a(13) =1617a(14) =2147a(15) =2380a(16) =3015a(17) =3290a(18) =4030a(19) =5017a(20) =7315a(21) =7740a(22) =8855a(23) =11310a(24) =13490a(25) =14950a(26) =15862a(27) =17120a(28) =18095a(29) =27270

External references