A089375 Duplicate of A069582.
31, 421, 71, 131, 191, 2551, 311, 391331, 431, 48241612864321, 911371, 971, 1031
Offset: 1
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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.
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 *)
For n = 9; a(9) = 391331 because A089374(9) = 39 and divisors of 39 are 1, 3, 13, 39; reverse concatenation of divisors A176558(21) = 391331 is noncomposite.
rcd:= proc(n) local D,T,i;
D:= sort(convert(numtheory:-divisors(n),list));
T:= D[1];
for i from 2 to nops(D) do
T:= T + 10^(1+ilog10(T))*D[i]
od;
T
end proc:
select(t -> t=1 or isprime(t), map(rcd, [$1..1000])); # Robert Israel, Aug 12 2020
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