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Search: id:A112888
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| A112888 |
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Least semiprime of a cluster of just n semiprimes. |
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+0
1
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OFFSET
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1,1
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COMMENT
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Clusters are sets composed of odd numbers. If we look at even numbers then the sequence would start 4,9,33 and terminates because in any group of four consecutive numbers greater than 4, 4 is a divisor to at least one member leaving a quotient greater than 1.
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EXAMPLE
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a(8)=8129 because 8129=11*739, 8131=47*173, 8133=3*2711, 8135=5*1627, 8137=79*103, 8139=3*2713, 8141=7*1163, 8143=17*479.
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MATHEMATICA
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spQ[n_] := Plus @@ Last /@ FactorInteger@n == 2; f[n_] := Block[{k = 1}, While[ s[[k]] + 2n != s[[k + n]] || s[[k]] + 2n + 2 == s[[k + n + 1]], k++ ]; s[[k]]]; s = {}; Do[ If[ spQ[n], AppendTo[s, n]], {n, 9, 7*10^6, 2}]; Table[ f[n], {n, 0, 7}]
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CROSSREFS
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Cf. A001358, A097824, A082919.
Sequence in context: A020228 A005939 A020326 this_sequence A048479 A031880 A036543
Adjacent sequences: A112885 A112886 A112887 this_sequence A112889 A112890 A112891
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KEYWORD
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more,nonn
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AUTHOR
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Robert G. Wilson v (rgwv(AT)rgwv.com), Nov 30 2005
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