A296814 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 = 7.

28738, 137042, 373763, 384767, 662426, 686627, 775595, 888227, 896498, 919115, 1216311, 1599578, 2022119, 2027195, 2218978, 2234018, 2276543, 2307491, 2375522, 2625351, 2920067, 3202863, 3329891, 3457887, 3568151, 3904634, 3917882, 4359491, 4468634, 4710863, 4773347

Offset

1,1

Links

Table of n, a(n) for n=1..31.

Example

28738 = 2*14369, 14369-2 = 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.

Maple

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, 8), i=1..2*10^3);

Crossrefs

Cf. A001358, A296096, A296808.

Sequence in context: A255659 A213460 A043660 * A259047 A237321 A249957

Adjacent sequences: A296811 A296812 A296813 * A296815 A296816 A296817

Keyword

nonn,easy

Author

Paolo P. Lava, Dec 21 2017

Status

approved