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A289259 Numbers k such that k^2 divides 2^k + 3^k. 1
1, 5, 55, 1971145, 3061355, 109715901845, 340799222665 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

If k is in the sequence and p is a prime factor, coprime to k, of 2^k + 3^k, then k*p is in the sequence.

55 = 5 * 11

1971145 = 5 * 11 * 35839

3061355 = 5 * 11 * 55661

109715901845 = 5 * 11 * 35839 * 55661

340799222665 = 5 * 11 * 55661 * 111323

See Known Terms link for additional terms.

From Felix Fröhlich, Jun 29 2017: (Start)

For k in the sequence, A220235(k) = 0.

Subsequence of A045576. (End)

LINKS

Table of n, a(n) for n=1..7.

Robert Israel and Ray Chandler, Known Terms

A. Velampalli et al, Mathematics StackExchange, Can you prove or disprove that there exist infinitely many integers n such that n^2 divides 2^n+3^n?

EXAMPLE

2^5 + 3^5 = 275 is divisible by 5^2, so 5 is in the sequence.

MAPLE

select(t -> 2&^t + 3&^t mod t^2 = 0, [$1..10^6]);

PROG

(PARI) is(n) = Mod(2, n^2)^n==-3^n \\ Felix Fröhlich, Jun 29 2017

(PARI) is(n) = Mod(2, n^2)^n+Mod(3, n^2)^n==0 \\ Charles R Greathouse IV, Jun 29 2017

CROSSREFS

Cf. A007689, A045576, A220235.

Sequence in context: A129440 A045729 A067515 * A072318 A174514 A041995

Adjacent sequences:  A289256 A289257 A289258 * A289260 A289261 A289262

KEYWORD

nonn,more

AUTHOR

Robert Israel, Jun 29 2017

EXTENSIONS

a(6)-a(7) confirmed as next terms by Ray Chandler, Jul 02 2017

Known terms updated and moved to a-file by Ray Chandler, Jul 03 2017

STATUS

approved

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Last modified October 16 13:06 EDT 2021. Contains 348041 sequences. (Running on oeis4.)