A089787 a(4n-3), a(4n-2), a(4n-1), and a(4n) are the units digit of the n-th prime followed by 1, 3, 7, and 9 respectively.

21, 23, 27, 29, 31, 33, 37, 39, 51, 53, 57, 59, 71, 73, 77, 79, 11, 13, 17, 19, 31, 33, 37, 39, 71, 73, 77, 79, 91, 93, 97, 99, 31, 33, 37, 39, 91, 93, 97, 99, 11, 13, 17, 19, 71, 73, 77, 79, 11, 13, 17, 19, 31, 33, 37, 39, 71, 73, 77, 79, 31, 33, 37, 39, 91, 93, 97, 99, 11

Offset: 1

Author

Roger L. Bagula, Jan 09 2004

Keywords

• nonn
• easy
• less

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A007652, A131712.

Cf. A089784 (essentially the same).

Programs

Mathematica

Flatten[Table[Mod[Prime[n], 10]*10+{1, 3, 7, 9}, {n, 1, 50}]]
a[n_] := Mod[Prime[Floor[(n + 3)/4]], 10]*10 + {1, 3, 7,
9}[[Mod[n - 1, 4] + 1]] (* Charles R Greathouse IV, Jan 02 2013 *)
Table[10*Mod[Prime[n],10]+{1,3,7,9},{n,30}]//Flatten (* Harvey P. Dale, Aug 24 2019 *)

PARI

a(n)=prime((n+3)\4)%10*10+[9,1,3,7][n%4+1] \\ Charles R Greathouse IV, Jan 02 2013

Formula

a(n) = 5+(-1)^n+3*cos(n*Pi/2)-3*sin(n*Pi/2)+10*(prime(floor((n+3)/4)) mod 10). - Wesley Ivan Hurt, May 06 2021