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A081943
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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.
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4
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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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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FORMULA
| 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) - Farideh Firoozbakht (mymontain(AT)yahoo.com), Apr 14 2004
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MAPLE
| 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}]
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CROSSREFS
| Cf. A073666, A081942.
Sequence in context: A171086 A096135 A092829 * A113004 A113001 A036812
Adjacent sequences: A081940 A081941 A081942 * A081944 A081945 A081946
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KEYWORD
| nonn
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AUTHOR
| Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Apr 02 2003
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EXTENSIONS
| More terms from Farideh Firoozbakht (mymontain(AT)yahoo.com), Apr 14 2004
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