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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
4, 6, 9, 21, 22, 25, 33, 39, 46, 49, 51, 54, 58, 78, 82, 93, 99, 111, 115, 121, 133, 141, 142, 147, 153, 154, 159, 162, 166, 169, 174, 177, 186, 187, 189, 201, 205, 219, 226, 235, 237, 247, 249, 253, 262, 267, 274, 286, 289, 291, 294, 301, 318 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000

EXAMPLE

   n |    divisors    | concatenation

  ---+----------------+--------------

   4 | (1) 2      (4) |        2

   6 | (1) 2, 3   (6) |       23

   9 | (1) 3      (9) |        3

  21 | (1) 3, 7  (21) |       37

  22 | (1) 2, 11 (22) |      211

  25 | (1) 5     (25) |        5

  33 | (1) 3, 11 (33) |      311

  39 | (1) 3, 13 (39) |      313

MAPLE

with(numtheory):

for k from 2 to 1000 do:

v0:=divisors(k):

nn:=nops(v0):

if nn > 2 then

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

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

v3:=parse(v2):

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

fi:

od: # Simon Plouffe

MATHEMATICA

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

CROSSREFS

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

Cf. A130139, A130140, A130141, A130142.

Sequence in context: A171127 A098485 A293399 * A115698 A039566 A032817

Adjacent sequences:  A120709 A120710 A120711 * A120713 A120714 A120715

KEYWORD

nonn,base

AUTHOR

Eric Angelini, Jul 19 2007

STATUS

approved

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Last modified December 9 17:48 EST 2018. Contains 318023 sequences. (Running on oeis4.)