This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
a(3)=479001599 because 479001599 is prime and equals 479001600-1 = A000142(12) - A000142(1) where 13 and 2 are prime numbers.
2 is a term because sigma(2) = 3. 3! - 1 = 5, a prime. 6 is a term because sigma(6) = 12. 12! - 1 = 479001599, a prime.
isok(n) = isprime(sigma(n)!-1); \\ Michel Marcus, Aug 07 2019
[n for n in range(1,150) if is_prime(factorial(sigma(n))-1)]
1973 and 9371 are respectively the smallest and the largest primes formed with the four digits that can end multidigit primes.
With[{w = Select[Range@ 10, GCD[#, 10] == 1 &]}, Select[FromDigits /@ Permutations[w, Length@ w], PrimeQ]] (* Michael De Vlieger, Feb 03 2019 *)
Select[FromDigits/@Flatten[Permutations/@Subsets[{1,3,7,9}],1],PrimeQ]//Union (* Harvey P. Dale, Apr 20 2025 *)
14 is a term, because 14!^2 + 13!^2 - 1 = 7600054456551997440000 + 38775788043632640000 - 1 = 7638830244595630079999 is a prime number.
select(k -> isprime((k^2+1)*((k-1)!)^2-1), [$1..1000]); # Robert Israel, Aug 12 2024
is(k) = isprime(k!^2 + (k-1)!^2 - 1);
from itertools import count, islice from sympy import isprime def A375310_gen(): # generator of terms f = 1 for k in count(1): if isprime((k**2+1)*f-1): yield k f *= k**2 A375310_list = list(islice(A375310_gen(),6)) # Chai Wah Wu, Oct 02 2024
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