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Take a squarefree semiprime and take the difference of its prime factors. If it is a squarefree semiprime repeat the process. Sequence lists the squarefree semiprimes that generate other squarefree semiprimes only in the first k steps of this process. Case k = 6.

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`%I #4 Jan 06 2018 22:04:06
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`%S 14367,16706,18663,28738,33551,49983,58407,62387,68519,77195,82743,
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`%T 97355,137042,137915,143223,149327,155114,181163,183818,214247,238715,
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`%U 262667,331211,332363,370343,373763,384767,405434,406743,448247,503567,503834,522983,576951,634115
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`%N Take a squarefree semiprime and take the difference of its prime factors. If it is a squarefree semiprime repeat the process. Sequence lists the squarefree semiprimes that generate other squarefree semiprimes only in the first k steps of this process. Case k = 6.
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`%e 14367 = 3*4789, 4789-3 = 4786 = 2*2393, 2393-2 = 2391 = 3*797, 797-3 = 794 = 2*397, 397-2 = 395 = 5*79, 79-5 = 74 = 2*37, 37-2 = 35 = 5*7 but 7-5 = 2 is not a squarefree semiprime.
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`%e 16706 = 2*8353, 8353-2 = 8351 = 7*1193, 1193-7 = 1186 = 2*593, 593-2 = 591 = 3*197, 197-3 = 194 = 2*97, 97-2 = 95 = 5*19, 19-5 = 14 = 2*7 but 7-2 = 5 is not a squarefree semiprime.
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`%p with(numtheory): P:=proc(n,h) local a,j,ok; ok:=1; a:=n; for j from 1 to h do if issqrfree(a) and nops(factorset(a))=2 then a:=ifactors(a)[2]; a:=a[1][1]-a[2][1]; else ok:=0; break; fi; od;if ok=1 then n; fi; end: seq(P(i,7),i=1..2*10^3);
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`%K nonn,easy
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`%O 1,1
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`%A _Paolo P. Lava_, Dec 21 2017
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