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A016825 Positive integers congruent to 2 mod 4: a(n) = 4*n+2, for n >= 0. 172
2, 6, 10, 14, 18, 22, 26, 30, 34, 38, 42, 46, 50, 54, 58, 62, 66, 70, 74, 78, 82, 86, 90, 94, 98, 102, 106, 110, 114, 118, 122, 126, 130, 134, 138, 142, 146, 150, 154, 158, 162, 166, 170, 174, 178, 182, 186, 190, 194, 198, 202, 206, 210, 214, 218, 222, 226, 230, 234 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

Twice the odd numbers, also called singly even numbers.

Numbers having equal numbers of odd and even divisors: A001227(a(n)) = A000005(2*a(n)). - Reinhard Zumkeller, Dec 28 2003

Continued fraction for coth(1/2) = (e+1)/(e-1). The continued fraction for tanh(1/2) = (e-1)/(e+1) would be a(0) = 0, a(n) = A016825(n-1), n >= 1.

No solutions to a(n) = b^2 - c^2. - Henry Bottomley, Jan 13 2001

Sequence gives n such that 8 is the largest power of 2 dividing A003629(k)^n-1 for any k. - Benoit Cloitre, Apr 05 2002

n such that Sum_{d|n}(-1)^d) = A048272(n) = 0. - Benoit Cloitre, Apr 15 2002

Also n such that Sum_{d|n} phi(d)*mu(n/d) = A007431(n) = 0. - Benoit Cloitre, Apr 15 2002

Also n such that Sum_{d|n}(d/A000005(d))*mu(n/d) = 0, n such that Sum_{d|n}(A000005(d)/d)*mu(n/d) = 0. - Benoit Cloitre, Apr 19 2002

Solutions to phi(x) = phi(x/2); primorial numbers are here. - Labos Elemer, Dec 16 2002

Together with 1, numbers that are not the leg of a primitive Pythagorean triangle. - Lekraj Beedassy, Nov 25 2003

For n>0: complement of A107750 and A023416(a(n)-1) = A023416(a(n)) <> A023416(a(n)+1). - Reinhard Zumkeller, May 23 2005

Also the minimal value of Sum_{i=1..n+2}(p(i) - p(i+1))^2, where p(n+3) = p(1), as p ranges over all permutations of {1,2,...,n+2} (see the Mihai reference). Example: a(2)=10 because the values of the sum for the permutations of {1,2,3,4} are 10 (8 times), 12 (8 times) and 18 (8 times). - Emeric Deutsch, Jul 30 2005

Except for a(n)=2, numbers having 4 as an anti-divisor. - Alexandre Wajnberg, Oct 02 2005

A139391(a(n)) = A006370(a(n)) = A005408(n). - Reinhard Zumkeller, Apr 17 2008

Also a(n) = (n-1) + n + (n+1) + (n+2), so a(n) and -a(n) are all the integers that are sums of four consecutive integers. - Rick L. Shepherd, Mar 21 2009

The denominator in Pi/8 = 1/2 - 1/6 + 1/10 - 1/14 + 1/18 - 1/22 + .... - Mohammad K. Azarian, Oct 13 2011

This sequence gives the positive zeros of i^x + 1 = 0, x real, where i^x = exp(i*x*Pi/2). - Ilya Gutkovskiy, Aug 08 2015

Numbers k such that Sum_{j=1..k} j^3 is not a multiple of k. - Chai Wah Wu, Aug 23 2017

Numbers k such that Lucas(k) is a multiple of 3. - Bruno Berselli, Oct 17 2017

Also numbers k such that t^k == -1 (mod 5), where t is a member of A047221. - Bruno Berselli, Dec 28 2017

The even numbers form a ring, and these are the primes in that ring. Note that unique factorization into primes does not hold, since 60 = 2*30 = 6*10. - N. J. A. Sloane, Nov 11 2019

REFERENCES

H. Bass, Mathematics, Mathematicians and Mathematics Education, Bull. Amer. Math. Soc. (N.S.) 42 (2004), no. 4, 417-430.

J. R. Goldman, The Queen of Mathematics, 1998, p. 70.

Granino A. Korn and Theresa M. Korn, Mathematical Handbook for Scientists and Engineers, McGraw-Hill Book Company, New York (1968).

LINKS

Harry J. Smith, Table of n, a(n) for n = 0..20000

A. Beiser, Concepts of Modern Physics, 2nd Ed., McGraw-Hill, 1973.

Daniel Forgues, Number of electrons per filled orbital [Link to an empty internal wiki page]

Tanya Khovanova, Recursive Sequences

D. H. Lehmer, Continued fractions containing arithmetic progressions, Scripta Mathematica, 29 (1973): 17-24. [Annotated copy of offprint]

I. Lukovits and D. Janezic, Enumeration of conjugated circuits in nanotubes, J. Chem. Inf. Comput. Sci. 44 (2004), 410-414.

Vasile Mihai and Michael Woltermann, Problem 10725: The Smoothest and Roughest Permutations, Amer. Math. Monthly, 108 (March 2001), pp. 272-273.

Paolo Emilio Ricci, Complex Spirals and Pseudo-Chebyshev Polynomials of Fractional Degree, Symmetry (2018) Vol. 10, No. 12, 671.

William A. Stein, The modular forms database

Eric Weisstein's World of Mathematics, Bishop Graph

Eric Weisstein's World of Mathematics, Maximal Clique

Eric Weisstein's World of Mathematics, Singly Even Number

Eric Weisstein's World of Mathematics, Square Number

G. Xiao, Contfrac

Index entries for continued fractions for constants

Index entries for linear recurrences with constant coefficients, signature (2,-1).

FORMULA

a(n) = 4*n + 2, for n >= 0.

a(n) = 2*A005408(n). - Lekraj Beedassy, Nov 28 2003

a(n) = A118413(n+1,2) for n>1. - Reinhard Zumkeller, Apr 27 2006

From Michael Somos, Apr 11 2007: (Start)

G.f.: 2*(1+x)/(1-x)^2.

E.g.f.: 2*(1+2*x)*exp(x).

a(n) = a(n-1) + 4.

a(-1-n) = -a(n). (End)

a(n) = 8*n - a(n-1) for n>0, a(0)=2. - Vincenzo Librandi, Nov 20 2010

From Reinhard Zumkeller, Jun 11 2012, Jun 30 2012 and Jul 20 2012: (Start)

A080736(a(n)) = 0. A007814(a(n)) = 1; A037227(a(n)) = 3. A214546(a(n)) = 0. (End)

a(n) = T(n+2) - T(n-2) where T(n) = n*(n+1)/2 = A000217(n). In general, if M(k,n) = 2*k*n + k, then M(k,n) = T(n+k) - T(n-k). - Charlie Marion, Feb 24 2020

EXAMPLE

0.4621171572600097585023184... = 0 + 1/(2 + 1/(6 + 1/(10 + 1/(14 + ...)))), i.e., CF for tanh(1/2).

2.1639534137386528487700040... = 2 + 1/(6 + 1/(10 + 1/(14 + 1/(18 + ...)))), i.e., CF for coth(1/2).

MAPLE

a := n -> 4*n+2: seq(a(n), n = 0 .. 70); # Stefano Spezia, Jun 17 2019

MATHEMATICA

Range[2, 280, 4] (* Vladimir Joseph Stephan Orlovsky, May 26 2011 *)

4*Range[0, 70] +2 (* Eric W. Weisstein, Dec 01 2017 *)

LinearRecurrence[{2, -1}, {2, 6}, 70] (* Eric W. Weisstein, Dec 01 2017 *)

CoefficientList[Series[2*(1+x)/(1-x)^2, {x, 0, 70}], x] (* Eric W. Weisstein, Dec 01 2017 *)

PROG

(MAGMA) [4*n+2 : n in [0..70]];

(PARI) a(n)= 4*n+2

(PARI) contfrac(tanh(1/2)) \\ To illustrate the 3rd comment. - Harry J. Smith, May 09 2009 [Edited by M. F. Hasler, Mar 09 2020]

(Haskell)

a016825 = (+ 2) . (* 4)

a016825_list = [2, 6 ..]  -- Reinhard Zumkeller, Feb 14 2012

(GAP) Flat(List([0..70], n->4*n+2)) # Stefano Spezia, Jun 17 2019

(Sage) [4*n+2 for n in (0..70)] # G. C. Greubel, Jun 28 2019

CROSSREFS

Cf. A107687. First differences of A001105.

Cf. A160327 (decimal expansion).

Subsequence of A042963.

Essentially the complement of A042965.

Sequence in context: A251538 A111284 A130824 * A161718 A122905 A132417

Adjacent sequences:  A016822 A016823 A016824 * A016826 A016827 A016828

KEYWORD

nonn,cofr,easy,nice

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified October 29 13:52 EDT 2020. Contains 338066 sequences. (Running on oeis4.)