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 A246947 a(1)=30; for n > 1, a(n) is the least integer not occurring earlier such that a(n) shares exactly three distinct prime divisors with a(n-1). 5
 30, 60, 90, 120, 150, 180, 210, 42, 84, 126, 168, 252, 294, 336, 378, 420, 70, 140, 280, 350, 490, 560, 630, 105, 315, 525, 735, 840, 240, 270, 300, 330, 66, 132, 198, 264, 396, 462, 154, 308, 616, 770, 110, 220, 440, 550, 660, 165, 495, 825, 990, 360, 390, 78 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All terms belong to A000977. Is this a permutation of A000977? - Michel Marcus, Nov 24 2015 LINKS Michel Lagneau, Table of n, a(n) for n = 1..2000 EXAMPLE 90 is in the sequence because the common prime distinct divisors between a(2)=60 and a(3)=90 are 2, 3 and 5. MAPLE with(numtheory):a0:={2, 3, 5}:lst:={}: for n from 1 to 100 do:   ii:=0:     for k from 30 to 50000 while(ii=0) do:       y:=factorset(k):n0:=nops(y):lst1:={}:         for j from 1 to n0 do:         lst1:=lst1 union {y[j]}:         od:          a1:=a0 intersect lst1:          if {k} intersect lst ={} and a1 <> {} and nops(a1)=3           then           printf(`%d, `, k):lst:=lst union {k}:a0:=lst1:ii:=1:          else          fi:       od:   od: MATHEMATICA f[s_List]:=Block[{m=s[[-1]], k=30}, While[MemberQ[s, k]||Intersection[Transpose[FactorInteger[k]][[1]], Transpose[FactorInteger[m]][[1]]]=={}|| Length[Intersection[Transpose[FactorInteger[k]][[1]], Transpose[FactorInteger[m]][[1]]]]!=3, k++]; Append[s, k]]; Nest[f, {30}, 70] PROG (PARI) lista(nn) = {a = 30; print1(a, ", "); fa = (factor(a)[, 1])~; va = [a]; k = 0; while (k!= nn, k = 1; while (!((#setintersect(fa, (factor(k)[, 1])~) == 3) && (! vecsearch(va, k))), k++); a = k; print1(a, ", "); fa = (factor(a)[, 1])~; va = vecsort(concat(va, k)); ); } \\ Michel Marcus, Nov 24 2015 CROSSREFS Cf. A000977, A064413, A246946. Sequence in context: A222618 A325992 A056954 * A226944 A249674 A050519 Adjacent sequences:  A246944 A246945 A246946 * A246948 A246949 A246950 KEYWORD nonn AUTHOR Michel Lagneau, Sep 08 2014 STATUS approved

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Last modified December 15 22:02 EST 2019. Contains 330012 sequences. (Running on oeis4.)