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A089374
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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).
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7
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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
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OFFSET
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1,2
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COMMENTS
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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
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LINKS
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EXAMPLE
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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.
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MAPLE
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select(n->isprime(parse(cat("", op(sort([op(numtheory[divisors](n))], `>`))))), [$1..3000])[]; (Alec Mihailovs, Aug 14 2005)
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MATHEMATICA
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Join[{1}, Select[Range[1000], PrimeQ[FromDigits[Flatten[IntegerDigits/@Reverse[Divisors[ #]]]]]&]] (* Harvey P. Dale, Feb 11 2024 *)
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CROSSREFS
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KEYWORD
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base,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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