A089787 - OEIS

%I #20 Dec 18 2022 01:41:30

%S 21,23,27,29,31,33,37,39,51,53,57,59,71,73,77,79,11,13,17,19,31,33,37,

%T 39,71,73,77,79,91,93,97,99,31,33,37,39,91,93,97,99,11,13,17,19,71,73,

%U 77,79,11,13,17,19,31,33,37,39,71,73,77,79,31,33,37,39,91,93,97,99,11

%N 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.

%H Harvey P. Dale, <a href="/A089787/b089787.txt">Table of n, a(n) for n = 1..1000</a>

%F 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

%t Flatten[Table[Mod[Prime[n], 10]*10+{1, 3, 7, 9}, {n, 1, 50}]]

%t a[n_] := Mod[Prime[Floor[(n + 3)/4]], 10]*10 + {1, 3, 7,

%t 9}[[Mod[n - 1, 4] + 1]] (* _Charles R Greathouse IV_, Jan 02 2013 *)

%t Table[10*Mod[Prime[n],10]+{1,3,7,9},{n,30}]//Flatten (* _Harvey P. Dale_, Aug 24 2019 *)

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

%Y Cf. A007652, A131712.

%Y Cf. A089784 (essentially the same).

%K nonn,easy,less

%O 1,1

%A _Roger L. Bagula_, Jan 09 2004