A084728 a(n) = smallest multiple of n such that a(1)*a(2)*...*a(n) + 1 is a prime.

1, 2, 3, 12, 25, 12, 7, 56, 36, 60, 11, 24, 104, 112, 225, 112, 102, 108, 38, 40, 231, 968, 23, 216, 600, 104, 486, 364, 174, 600, 310, 288, 132, 102, 140, 252, 740, 760, 312, 40, 656, 588, 4429, 1452, 3690, 552, 94, 96, 2744, 1000, 1530, 1664, 2279, 1512, 5005

Offset

1,2

Comments

The partial product is a multiple of n!.

Links

Table of n, a(n) for n=1..55.

Example

a(5) = 25 as 1*2*3*12*25 + 1= 1801 is a prime but 1*2*3*12*k + 1 is not a prime for k = 5,10,15 and 20.

Mathematica

a[1] = 1; a[n_] := a[n] = Block[{k = 1, pp = Product[a[i], {i, 1, n - 1}]}, While[ !PrimeQ[ pp*k*n + 1], k++ ]; k*n]; Table[ a[n], {n, 1, 54}]

Crossrefs

Cf. A084729.

Sequence in context: A107928 A130337 A293697 * A099429 A049601 A148060

Adjacent sequences: A084725 A084726 A084727 * A084729 A084730 A084731

Keyword

nonn

Author

Amarnath Murthy and Meenakshi Srikanth (menakan_s(AT)yahoo.com), Jun 13 2003

Extensions

Edited and extended by Robert G. Wilson v, Jun 15 2003

Status

approved