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 A120712 Numbers n with the property that the concatenation of the proper divisors of n (i.e., excluding 1 and n) is a prime. 9

%I

%S 4,6,9,21,22,25,33,39,46,49,51,54,58,78,82,93,99,111,115,121,133,141,

%T 142,147,153,154,159,162,166,169,174,177,186,187,189,201,205,219,226,

%U 235,237,247,249,253,262,267,274,286,289,291,294,301,318

%N Numbers n with the property that the concatenation of the proper divisors of n (i.e., excluding 1 and n) is a prime.

%H T. D. Noe, <a href="/A120712/b120712.txt">Table of n, a(n) for n = 1..1000</a>

%e n | divisors | concatenation

%e ---+----------------+--------------

%e 4 | (1) 2 (4) | 2

%e 6 | (1) 2, 3 (6) | 23

%e 9 | (1) 3 (9) | 3

%e 21 | (1) 3, 7 (21) | 37

%e 22 | (1) 2, 11 (22) | 211

%e 25 | (1) 5 (25) | 5

%e 33 | (1) 3, 11 (33) | 311

%e 39 | (1) 3, 13 (39) | 313

%p with(numtheory):

%p for k from 2 to 1000 do:

%p v0:=divisors(k):

%p nn:=nops(v0):

%p if nn > 2 then

%p v1:=[seq(v0[j],j=2..nn-1)]:

%p v2:=cat(seq(convert(v1[n],string),n=1..nops(v1))):

%p v3:=parse(v2):

%p if isprime(v3) = true then lprint(k,v3) fi:

%p fi:

%p od: # _Simon Plouffe_

%t fQ[n_] := PrimeQ@ FromDigits@ Most@ Rest@ Divisors@ n; Select[ Range[2, 320], fQ]

%Y Cf. A037274, A037278, A037279, A106708, A120713, A120716.

%Y Cf. A130139, A130140, A130141, A130142.

%K nonn,base

%O 1,1

%A _Eric Angelini_, Jul 19 2007

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Last modified October 22 10:57 EDT 2018. Contains 316437 sequences. (Running on oeis4.)