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A046424
Numbers requiring 4 steps to reach a prime under the prime factor concatenation procedure.
1
10, 15, 16, 26, 27, 95, 118, 119, 121, 132, 145, 202, 204, 217, 286, 295, 302, 308, 354, 361, 370, 377, 400, 437, 451, 453, 455, 471, 474, 483, 506, 518, 526, 528, 530, 545, 567, 600, 610, 616, 635, 649, 650, 654, 655, 672, 699, 700, 718, 720, 722, 735, 745
OFFSET
1,1
LINKS
Patrick De Geest, Home Primes
EXAMPLE
95 = 5*19 -> 519 = 3*173 -> 3173 = 19*167 -> 19167 = 3*6389 and 36389 is prime.
MAPLE
filter:= proc(n) local k, F, x, j;
x:= n;
for k from 1 to 4 do
if isprime(x) then return false fi;
F:= sort(ifactors(x)[2], (a, b) -> a[1]<b[1]);
F:= map(t -> t[1] $ t[2], F);
x:= F[1];
for j from 2 to nops(F) do
x:= x*10^(1+ilog10(F[j]))+F[j]
od;
od;
isprime(x)
end proc:
select(filter, [$2..1000]); # Robert Israel, Jun 25 2019
CROSSREFS
Sequence in context: A253594 A330698 A342593 * A076226 A122435 A091049
KEYWORD
nonn,base
AUTHOR
Patrick De Geest, Jun 15 1998
EXTENSIONS
Edited by Charles R Greathouse IV, Apr 28 2010
STATUS
approved