

A112888


Least semiprime of a cluster of just n semiprimes.


1




OFFSET

1,1


COMMENTS

Clusters are sets composed of odd numbers.
If we include 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.
Any set of 9 consecutive odd numbers contain a multiple of 9, which not semiprime (unless it is equal to 9). Hence there are no 9 consecutive odd semiprimes.


LINKS



EXAMPLE

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.


MATHEMATICA

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}]
Join[{9}, Module[{osps=Select[Range[9, 10001, 2], PrimeOmega[#]==2&]}, #[[2]]& /@ Table[ SelectFirst[Partition[osps, n+2, 1], Union[ Differences[ Rest[ Most[#]]]]=={2}&&Last[#]#[[2]]!=2&&#[[2]]#[[1]]!=2&], {n, 2, 8}]]] (* Harvey P. Dale, Jun 01 2016 *)


CROSSREFS



KEYWORD

nonn,fini,full


AUTHOR



EXTENSIONS



STATUS

approved



