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 A181623 Numbers starting with 1 such that the sum of any two distinct elements has four distinct prime factors. 0
 1, 209, 1121, 2989, 11381, 34889, 47701, 62453, 188785, 878185, 1761737, 3931385, 5630905, 7990481, 32892077, 204570037, 253223785, 1353794333, 2877954833 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Choose the first number not leading to a contradiction. LINKS EXAMPLE The subset {1, 209, 1121} gives the products of four distinct prime factors such   that  {2*3*5*7,  2*3*11*17,  2*3*5*137}. MAPLE with(numtheory):nn:=100000:T:=array(1..nn): U:=array(1..nn): for p from 1 to   nn do: T[p]:=p:U[p]:=1:od:for u from 1 to 30 do: k:=1+u:for n from u+1 to nn   do:s:=T[n]+T[u]:s1:=nops(factorset(s)):s2:=bigomega(s):if s1=4 and s2=4 then   U[k]:=T[n]:k:=k+1:else fi:od:for i from 1 to nn do:T[i]:=U[i]:od:od:for j from   1 to 30 do:printf(`%d, `, T[j]):od: CROSSREFS Cf. A181620, A181622, A180615, A180565, A180514. Sequence in context: A211672 A203048 A203041 * A104874 A157441 A203040 Adjacent sequences:  A181620 A181621 A181622 * A181624 A181625 A181626 KEYWORD nonn AUTHOR Michel Lagneau, Jan 31 2011 EXTENSIONS a(9)-a(19) from Donovan Johnson, Feb 14 2011 STATUS approved

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