

A015950


Numbers n such that n  4^n + 1.


5



1, 5, 25, 125, 205, 625, 1025, 2525, 3125, 5125, 8405, 12625, 15625, 25625, 42025, 63125, 78125, 103525, 128125, 168305, 202525, 210125, 255025, 315625, 344605, 390625, 517625, 640625, 841525, 875125, 1012625, 1050625, 1275125
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OFFSET

1,2


COMMENTS

From Robert Israel, Sep 14 2017: (Start)
All terms except 1 are congruent to 5 mod 20.
If n is a term and prime p  n, then n*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 2m where m is in the sequence, then m*p is in the sequence. (End)


LINKS

Robert Israel, Table of n, a(n) for n = 1..325


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


PROG

(PARI) isok(n) = Mod(4, n)^n == 1; \\ Michel Marcus, Sep 15 2017


CROSSREFS

Cf. A015945.
Sequence in context: A271380 A036149 A061974 * A267780 A228736 A126642
Adjacent sequences: A015947 A015948 A015949 * A015951 A015952 A015953


KEYWORD

nonn,changed


AUTHOR

Robert G. Wilson v


STATUS

approved



