A081943 a(1) = 1, a(n)= smallest number not occurring earlier such that a(n-1)*a(n) -1 is a prime. re-arrangement of natural numbers such that the product of adjacent terms is one more than a prime.

1, 3, 2, 4, 5, 6, 7, 12, 9, 8, 10, 11, 18, 13, 14, 16, 15, 24, 20, 19, 30, 17, 22, 21, 28, 23, 36, 27, 26, 33, 40, 32, 31, 42, 25, 50, 34, 38, 39, 48, 43, 60, 35, 46, 45, 44, 37, 54, 41, 52, 57, 56, 55, 68, 49, 62, 64, 53, 70, 29, 66, 63, 76, 47, 82, 69, 58, 51, 72, 61

Offset: 1

Author

Amarnath Murthy, Apr 02 2003

Keywords

• nonn

Links

• Ivan Neretin, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A073666, A081942.

Programs

Mathematica

a[1]=1; a[n_]:=a[n]=(For[c=Sort[Table[a[k],{k,n-1}]]; d=Append[c,Last[c]+1]; m=First[Complement[Range[Last[d]],c]], MemberQ[c,m]||!PrimeQ[m*a[n-1]-1],m++ ]; m); Table[a[k],{k,70}] (* Farideh Firoozbakht, Apr 14 2004 *)

Extensions

More terms from Farideh Firoozbakht, Apr 14 2004