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A175354 Numbers m such that reverse concatenations of divisors of m are nonprimes. 4
1, 2, 5, 6, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 20, 21, 22, 23, 24, 26, 27, 28, 29, 30, 32, 33, 34, 35, 36, 37, 38, 40, 41, 42, 44, 45, 46, 47, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

See A176558(n) = reverse concatenation of divisors of n. See A176588 for corresponding values of reverse concatenations. Complement of A089374(n) for n >= 2.

LINKS

Table of n, a(n) for n=1..71.

EXAMPLE

Divisors of 12: 1, 2, 3, 4, 6, 12; reverse concatenation of divisors 1264321 is nonprime number.

MAPLE

filter:= proc(n) local r, L, i;

  L:= sort(convert(numtheory:-divisors(n), list));

  r:= L[1];

  for i from 2 to nops(L) do

   r:= r + 10^(1+ilog10(r))*L[i]

  od;

  not isprime(r)

end proc:

select(filter, [$1..100]); # Robert Israel, Apr 21 2020

PROG

(PARI) rcd(n)=my(d=divisors(n)); n=1; for(i=2, #d, n=glue(d[i], n)); n

glue(a, b)=a*10^#Str(b)+b

for(m=1, 81, if(!isprime(rcd(m)), print1(m", ")))

CROSSREFS

Cf. A089374, A176558, A176588.

Sequence in context: A159752 A228089 A126971 * A224778 A286727 A090946

Adjacent sequences:  A175351 A175352 A175353 * A175355 A175356 A175357

KEYWORD

nonn,base

AUTHOR

Jaroslav Krizek, Apr 20 2010

EXTENSIONS

Program, editing, and extension by Charles R Greathouse IV, Apr 23 2010

Corrected by Jaroslav Krizek, Apr 26 2010

STATUS

approved

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Last modified February 28 11:45 EST 2021. Contains 341703 sequences. (Running on oeis4.)