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A191647 Numbers n with property that the concatenation of their anti-divisors is a prime. 3
3, 4, 5, 10, 14, 16, 40, 46, 100, 145, 149, 251, 340, 373, 406, 424, 439, 466, 539, 556, 571, 575, 617, 619, 628, 629, 655, 676, 689, 724, 760, 779, 794, 899, 901, 941, 970, 989, 1019, 1055, 1070, 1076, 1183, 1213, 1226, 1231, 1258, 1270, 1285, 1331, 1340 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Similar to A120712 which uses the proper divisors of n.

LINKS

Klaus Brockhaus Table of n, a(n) for n = 1..1000 (first 250 terms from Paolo P. Lava)

EXAMPLE

The anti-divisors of 40 are 3, 9, 16, 27, and 391627 is prime, hence 40 is in the sequence.

MAPLE

P:=proc(i) local a, b, c, d, k, n, s, v; v:=array(1..200000);

for n from 3 by 1 to i do k:=2; b:=0;

while k<n do if (k mod 2)=0  then

     if (n mod k)>0 and (2*n mod k)=0 then b:=b+1; v[b]:=k; fi;

     else

     if (n mod k)>0 and (((2*n-1) mod k)=0 or ((2*n+1) mod k)=0) then

b:=b+1; v[b]:=k; fi; fi; k:=k+1; od; a:=v[1];

for s from 2 to b do a:=a*10^floor(1+evalf(log10(v[s])))+v[s]; od;

if isprime(a) then print(n); fi;

od; end: P(10^6);

MATHEMATICA

antiDivisors[n_Integer] := Cases[Range[2, n - 1], _?(Abs[Mod[n, #] - #/2] < 1 &)]; a191647[n_Integer] := Select[Range[n],

PrimeQ[FromDigits[Flatten[IntegerDigits /@ antiDivisors[#]]]] &]; a191647[1350] (* Michael De Vlieger, Aug 09 2014, "antiDivisors" after Harvey P. Dale at A066272 *)

PROG

(Python)

from sympy import isprime

[n for n in range(3, 10**4) if isprime(int(''.join([str(d) for d in range(2, n) if n%d and 2*n%d in [d-1, 0, 1]])))] # Chai Wah Wu, Aug 08 2014

CROSSREFS

Cf. A037279, A066272, A106708, A120712, A120716, A191648.

Sequence in context: A260823 A135114 A240789 * A195131 A240793 A079351

Adjacent sequences:  A191644 A191645 A191646 * A191648 A191649 A191650

KEYWORD

nonn,base

AUTHOR

Paolo P. Lava, Jun 10 2011

EXTENSIONS

a(618) corrected in b-file by Paolo P. Lava, Feb 28 2018

STATUS

approved

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Last modified January 23 19:45 EST 2020. Contains 331175 sequences. (Running on oeis4.)