A056129 Final nonzero digit of n-th primorial.

1, 2, 6, 3, 1, 1, 3, 1, 9, 7, 3, 3, 1, 1, 3, 1, 3, 7, 7, 9, 9, 7, 3, 9, 1, 7, 7, 1, 7, 3, 9, 3, 3, 1, 9, 1, 1, 7, 1, 7, 1, 9, 9, 9, 7, 9, 1, 1, 3, 1, 9, 7, 3, 3, 3, 1, 3, 7, 7, 9, 9, 7, 1, 7, 7, 1, 7, 7, 9, 3, 7, 1, 9, 3, 9, 1, 3, 7, 9, 9, 1, 9, 9, 9, 7, 3, 9, 1, 7, 7, 1, 7, 3, 1, 1, 9, 7, 3, 3, 9, 9

Offset: 0

Author

Robert G. Wilson v, Jul 28 2000

Keywords

• easy
• nonn
• base

Comments

For n > 2, a(n) is in {1, 3, 7, 9}.

Links

• Index entries for sequences related to final digits of numbers

Crossrefs

Cf. A002110, A008904.

Programs

Mathematica

Do[p = Product[Prime[m], {m, 1, n}]; While[Mod[p, 10] == 0, p = p/10]; Print[Mod[p, 10]], {n, 0, 100}]

PARI

a(n)=if(n<3, return([1,2,6][n+1])); my(m=Mod(3,10)); forprime(p=7,prime(n), m*=p); lift(m) \\ Charles R Greathouse IV, Dec 03 2024

Formula

a(n) = prime(n)#/10 mod 10 for n > 2. - Charles R Greathouse IV, Dec 03 2024