cp's OEIS Frontend

For a background discussion of dispersions and their fractal sequences, see A191426. For dispersions of congruence sequences mod 3, mod 4, or mod 5, see A191655, A191663, A191667, A191702.

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Suppose that {2,3,4,5,6} is partitioned as {x1, x2} and {x3,x4,x5}. Let S be the increasing sequence of numbers >1 and congruent to x1 or x2 mod 5, and let T be the increasing sequence of numbers >1 and congruent to x3 or x4 or x5 mod 5. There are 10 sequences in S, each matched by a (nearly) complementary sequence in T. Each of the 20 sequences generates a dispersion, as listed here:

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A191722=dispersion of A008851 (0, 1 mod 5 and >1)

A191723=dispersion of A047215 (0, 2 mod 5 and >1)

A191724=dispersion of A047218 (0, 3 mod 5 and >1)

A191725=dispersion of A047208 (0, 4 mod 5 and >1)

A191726=dispersion of A047216 (1, 2 mod 5 and >1)

A191727=dispersion of A047219 (1, 3 mod 5 and >1)

A191728=dispersion of A047209 (1, 4 mod 5 and >1)

A191729=dispersion of A047221 (2, 3 mod 5 and >1)

A191730=dispersion of A047211 (2, 4 mod 5 and >1)

A191731=dispersion of A047204 (3, 4 mod 5 and >1)

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A191732=dispersion of A047202 (2,3,4 mod 5 and >1)

A191733=dispersion of A047206 (1,3,4 mod 5 and >1)

A191734=dispersion of A032793 (1,2,4 mod 5 and >1)

A191735=dispersion of A047223 (1,2,3 mod 5 and >1)

A191736=dispersion of A047205 (0,3,4 mod 5 and >1)

A191737=dispersion of A047212 (0,2,4 mod 5 and >1)

A191738=dispersion of A047222 (0,2,3 mod 5 and >1)

A191739=dispersion of A008854 (0,1,4 mod 5 and >1)

A191740=dispersion of A047220 (0,1,3 mod 5 and >1)

A191741=dispersion of A047217 (0,1,2 mod 5 and >1)

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For further information about these 20 dispersions, see A191722.

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Regarding the dispersions A191722-A191741, there are general formulas for sequences of the type "(a or b mod m)" and "(a or b or c mod m)" used in the relevant Mathematica programs.

A119617 is union of A134153 and A134154. A000538(n) is divisible by A000330(n) if and only n is congruent to {1, 3} mod 5 (see A047219). A134154(n) is case when n is congruent to 1 mod 5 see cases 2)

A119617 is union of A134153 and A134154 A000538(n) is divisible by A000330(n) if and only n is congruent to {1, 3} mod 5 (see A047219) A134154(n) is case when n is congruent to 3 mod 5 see cases 2)

Theorem (Sierpinski, 1963): n is a term iff n^2+(n+1)^2 is a composite number. - N. J. A. Sloane, Feb 29 2020

For n > 1, A047219 is a subset of this sequence. This is because n^2 + (n+1)^2 is divisible by 5 if n is (1 or 3) mod 5 (also see A027861). - Dmitry Kamenetsky, Sep 02 2008

From Hermann Stamm-Wilbrandt, Sep 10 2014: (Start)

For n > 0, A212160 is a subset of this sequence (n^2 + (n+1)^2 is divisible by 13 if n == (2 or 10) (mod 13)).

For n >= 0, A212161 is a subset of this sequence (n^2 + (n+1)^2 is divisible by 17 if n == (6 or 10) (mod 17)).

The above are for divisibility by 5, 13, 17; notation (1,3,5), (2,10,13), (6,10,17). Divisibility by p for a and p-a-1; notation (a,p-a-1,p). These are the next tuples: (8,20,29), (15,21,37), (4,36,41), (11,41,53), ... . The corresponding sequences are a subset of this sequence (8,20,37,49,66,78,... for (8,20,29)). These sequences have no entries in the OEIS yet. For any prime of the form 4*k+1 there is exactly one of these tuples/sequences.

For n > 1, A000217 (triangular numbers) is a subset of this sequence (3,6,10,15,...); z=A000217(n), y=z-1, x=n.

For n > 0, A001652 is a subset of this sequence; z=A001652(n), x=y=A053141(n).

For n > 1, A001108(=A115598) is a subset of this sequence; z=A001108(n), x=A076708(n), y=x+1.

For n > 0, A124124(2*n+1)(=A098790(2*n)) is a subset of this sequence (6,37,218,...); z=A124124(2*n+1), x=a(n)-1, y=a(n)+1, a(m) = 6*a(m-1) - a(m-2) + 2, a(0)=0, a(1)=4.

(End)

This sequence is identical to the sequence of numbers of the form k = a^2 + b, where a and b are positive integers and b is a factor of 2a greater than 1, in which case the continued fraction expansion of sqrt(k) is [a; [2a/b, 2a]]. - David Terr, Jun 11 2004

Number of partitions of n into distinct parts congruent to 1 or 3 mod 5.

A001844(N) = N^2 + (N+1)^2 = 4*A000217(N) + 1 is divisible by 13 if and only if N=a(n), n>=0. For the proof it suffices to show that only N=2 and N=10 from {0,1,..,12} satisfy A001844(N)== 0 (mod 13). Note that only primes of the form p= 4*k+1 (A002144) can be divisors of A001844 (see a Wolfdieter Lang comment there giving the reference).

Partial sums of the sequence [2,5,8,5,8,5,8,5,8,...] (see the o.g.f., and subtract 2 to see the 5,8 periodicity).

A001844(N) = N^2 + (N+1)^2 = 4*A000217(N) + 1 is divisible by 17 if and only if N = a(n), n >= 0. For the proof it suffices to show that only N=6 and N=10 from {0,1,...,16} satisfy A001844(N) == 0 (mod 17). Note that only primes of the form p = 4*k+1 (A002144) can be divisors of A001844 (see a Wolfdieter Lang comment there giving the reference). Note also that if N^2 + (N+1)^2 == 0 (mod p), with any prime p (necessarily from A002144), then also p-1-N satisfies this congruence. This explains why 10 = 17-1-6 is the (incongruent) companion of 6.

Partial sums of the sequence 6,4,13,4,13,4,13,4,13,4,13,... (see the o.g.f., and subtract 6 to see the remaining 4, 13=17-4 periodicity).

Also zero followed by bisection (even part) of A088003.

Numbers ending in 0, 2 or 4 (cf. 2*A053796(n)). Therefore we can easily see that a(m)^(2*k+1)==-1 (mod 5) only for m in A047219, while a(m)^(2*k)==-1 (mod 5) only for m in A016873 and k odd.

From Vincenzo Librandi, Nov 16 2012: (Start)

First column: A007494(n+1);

second column: A047219(n+2);

third column: A047383(n+3);

fourth column: A193910(n+4).

Conjecture: If h does not belong to the sequence, then 4*h+1 is prime. (End)