OFFSET
1,2
COMMENTS
From Robert Israel, Sep 14 2017: (Start)
All terms except 1 are congruent to 5 mod 20.
If k is a term and prime p | k, then k*p is a term.
All prime factors of terms == 1 (mod 4).
If p is a prime == 1 (mod 4) and the order of 4 (mod p) is 2*m where m is in the sequence, then m*p is in the sequence. (End)
LINKS
Max Alekseyev, Table of n, a(n) for n = 1..3514 (first 325 terms from Robert Israel)
EXAMPLE
4^5 + 1 = 1025 and 1025 is divisible by 5, so 5 is a term.
MAPLE
select(n -> 4 &^ n + 1 mod n = 0, [1, seq(i, i=5..10^7, 20)]); # Robert Israel, Sep 14 2017
MATHEMATICA
Select[Prepend[20 Range[0, 10^5] + 5, 1], Mod[4^# + 1, #] == 0 &] (* Michael De Vlieger, Dec 31 2018 *)
PROG
(PARI) is_A015950(n) = Mod(4, n)^n == -1; \\ Michel Marcus, Sep 15 2017
(Magma) [n: n in [1..10^6] | Modexp(4, n, n)+1 eq n]; // Jinyuan Wang, Dec 29 2018
(Python)
A015950_list = [n for n in range(1, 10**6) if pow(4, n, n) == n-1] # Chai Wah Wu, Mar 25 2021
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved