A175355 -id:A175355 - 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

A089374 Numbers n such that the concatenation (in descending order) of all the divisors of n, with 1 in the least significant position, is prime (or 1).

Original entry on oeis.org

1, 3, 4, 7, 13, 19, 25, 31, 39, 43, 48, 91, 97, 103, 109, 117, 151, 157, 181, 193, 211, 241, 244, 247, 271, 289, 292, 301, 309, 325, 337, 349, 367, 388, 409, 421, 439, 487, 523, 547, 571, 597, 601, 613, 628, 631, 633, 687, 691, 703, 711, 733, 769, 772, 793, 811

Offset: 1

Amarnath Murthy, Nov 08 2003

See A176558(n) = reverse concatenation of divisors of n. See A175355 for corresponding values of reverse concatenations. Complement of A175354(n) for n >= 2. - Jaroslav Krizek, Apr 20 2010

If prime p divides n, then the exponent of p in the prime factorization of n is odd if p == 1 (mod 3) and even if p == 2 (mod 3). In particular, the sequence has no terms == 2 (mod 3). - Robert Israel, Apr 21 2020

			4 is a term as 421 is prime; 39 is a term as concatenation of 39,13,3 and 1, i.e. 391331, is prime.
25 is a member as 2551 is prime.
Divisors of 39 are 1,3,13,39; reverse concatenation of divisors 391331 is prime.
48 is a member as 48241612864321 is a prime.

Robert Israel, Table of n, a(n) for n = 1..10000

Cf. A069582, A323427 (primes p such that p^2 is in the sequence).

select(n->isprime(parse(cat("",op(sort([op(numtheory[divisors](n))],`>`))))),[$1..3000])[]; (Alec Mihailovs, Aug 14 2005)

Join[{1},Select[Range[1000],PrimeQ[FromDigits[Flatten[IntegerDigits/@Reverse[Divisors[ #]]]]]&]] (* Harvey P. Dale, Feb 11 2024 *)

Corrected and extended by David Wasserman, Sep 15 2005

Edited by N. J. A. Sloane, Apr 29 2007, Aug 14 2010

cp's OEIS Frontend

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

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