A007395 Constant sequence: the all 2's sequence. (Formerly M0208)

2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2

Offset

1,1

Comments

Continued fraction for 1 + sqrt(2). - Philippe Deléham, Nov 14 2006

a(n) = A213999(n,1). - Reinhard Zumkeller, Jul 03 2012

The least witness function W(k) is defined for odd composite numbers k. The sequence W(k) does not have its own entry in the OEIS because W(k) = 2 for all k with 9 <= k < 2047; then W(2047)=3. Cf. A089105. - N. J. A. Sloane, Sep 17 2014

a(n) = A254858(n-1,1). - Reinhard Zumkeller, Feb 09 2015

a(n) = number of permutations of length n+2 having exactly one ascent such that the first element the permutation is 2. - Ran Pan, Apr 20 2015

With alternating signs, this is the sequence of determinants of the 3 X 3 matrices m with m(i,j) = Fibonacci(n+i+j-2)^2. - Michel Marcus, Dec 23 2015

For p = prime(n+2), a(n) = ord_p(H_(p-1)), where ord_p denotes the p-adic valuation and H_i = 1 + 1/2 + ... + 1/i is a harmonic sum, except for n = 1944 and n = 157504, where ord_p(H_(p-1)) = 3, and any other term of A088164 that may exist (see Conrad link). The sequence a(n) = ord_p(H_(p-1)) does not have its own entry in the OEIS. - Felix Fröhlich, Mar 16 2016

This sequence is the only infinite bounded sequence of positive integers such that a(n) = (a(n-1) + a(n-2)) / gcd(a(n-1), a(n-2)) for all n >= 2. - Bernard Schott, Dec 28 2018

References

Titu Andreescu and Dorin Andrica, Number Theory, Birkhäuser, 2009, from 1999 Russian Mathematical Olympiad, p. 347.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Links

Table of n, a(n) for n=1..102.

25th All-Russian Mathematical Olympiad, Grade 10, Problem 2, p. 2, 1999.

Tobias Boege and Thomas Kahle, Construction Methods for Gaussoids, arXiv:1902.11260 [math.CO], 2019.

K. Conrad, The p-adic growth of harmonic sums

Daniele A. Gewurz and Francesca Merola, Sequences realized as Parker vectors of oligomorphic permutation groups, J. Integer Seqs., Vol. 6, 2003.

R. H. Hardin, Binary arrays with both rows and cols sorted, symmetries

Tanya Khovanova, Recursive Sequences

Aram Tangboonduangjit and Thotsaporn Thanatipanonda, Determinants Containing Powers of Generalized Fibonacci Numbers, arXiv:1512.07025 [math.CO], 2015.

Eric Weisstein's World of Mathematics, Hamiltonian Cycle

Index to divisibility sequences

Index entries for recurrences a(n) = k*a(n - 1) +/- a(n - 2)

Index entries for linear recurrences with constant coefficients, signature (1).

Formula

G.f.: 2/(1-x), and e.g.f.: 2*e^x. - Mohammad K. Azarian, Dec 22 2008

a(n) = A000005(A000040(n)). - Omar E. Pol, Feb 28 2018

a(n) = A002061(n) - A165900(n). - Torlach Rush, Feb 21 2019

Mathematica

Table[2, {105}]

Prog

(PARI) a(n) = 2 \\ Charles R Greathouse IV, Apr 07 2012
(Haskell)
a007395 = const 2
a007395_list = repeat 2  -- Reinhard Zumkeller, May 07 2012
(Maxima) makelist(2, n, 0, 30); /* Martin Ettl, Nov 09 2012 */
(Python)
def A007395(n): return 2 # Chai Wah Wu, Nov 10 2022

Crossrefs

Cf. A000004, A000012, A002061, A010701, A089105, A165900.

Cf. A213999, A254858.

Sequence in context: A130130 A046698 A211662 * A036453 A040000 A211665

Adjacent sequences: A007392 A007393 A007394 * A007396 A007397 A007398

Keyword

nonn,easy

Author

N. J. A. Sloane

Status

approved