A176588 -id:A176588 - cp's OEIS frontend

A176558 a(n) is the reverse concatenation of divisors of n.

1, 21, 31, 421, 51, 6321, 71, 8421, 931, 10521, 111, 1264321, 131, 14721, 15531, 168421, 171, 1896321, 191, 20105421, 21731, 221121, 231, 2412864321, 2551, 261321, 27931, 28147421, 291, 30151065321, 311, 32168421, 331131, 341721, 35751, 361812964321, 371

Offset: 1

Jaroslav Krizek, Apr 20 2010

Union of A089374(n) for n >= 1 and A175354(n) for n >= 2. - Jonathan Vos Post, Apr 17 2011

			For n=12; divisors of 12: 1,2,3,4,6,12; a(12)=1264321 (reverse concatenation).

Indranil Ghosh, Table of n, a(n) for n = 1..50000

Cf. A089374, A175354, A175355, A176588, A176589.

a:= n-> parse(cat(sort([numtheory[divisors](n)[]], `>`)[])):
seq(a(n), n=0..40);  # Alois P. Heinz, Dec 31 2020

Table[FromDigits@ Flatten@ Map[IntegerDigits, Reverse@ Divisors@ n], {n, 34}] (* Michael De Vlieger, Jan 23 2017 *)

a(n) = {s = ""; fordiv(n, d, s = concat(Str(d), s)); eval(s);} \\ Michel Marcus, Feb 16 2015

from sympy import divisors
def a(n): return int("".join(str(d) for d in divisors(n)[::-1]))
print([a(n) for n in range(1, 35)]) # Michael S. Branicky, Dec 31 2020

More terms from Charles R Greathouse IV, Apr 23 2010
Corrected by Jaroslav Krizek, Apr 26 2010
Edited by Charles R Greathouse IV, Apr 30 2010

A175354 Numbers m such that reverse concatenations of divisors of m are nonprimes.

Original entry on oeis.org

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

Offset: 1

Jaroslav Krizek, Apr 20 2010

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

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

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Cf. A089374, A176558, A176588.

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

Select[Range[90],!PrimeQ[FromDigits[Flatten[IntegerDigits/@Reverse[Divisors[#]]]]]&] (* Harvey P. Dale, Mar 21 2025 *)

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", ")))

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

Corrected by Jaroslav Krizek, Apr 26 2010

cp's OEIS Frontend

A176558 a(n) is the reverse concatenation of divisors of n.

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