A181623 Sequence starting with 1 such that the sum of any two distinct elements has four distinct prime factors.
1, 209, 1121, 2989, 11381, 34889, 47701, 62453, 188785, 878185, 1761737, 3931385, 5630905, 7990481, 32892077, 204570037, 253223785, 1353794333, 2877954833
Offset: 1
Keywords
Examples
Each of the three pairwise sums of the subset {1, 209, 1121} is the product of four distinct prime factors: {2*3*5*7, 2*3*11*17, 2*3*5*137}.
Programs
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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:
Extensions
a(9)-a(19) from Donovan Johnson, Feb 14 2011
Comments