login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A138749 a(n) = 2*a(n-1) - 5*a(n-2). 1
-1, -7, -9, 17, 79, 73, -249, -863, -481, 3353, 9111, 1457, -42641, -92567, 28071, 518977, 897599, -799687, -6087369, -8176303, 14084239, 69049993, 67678791, -209892383, -758178721, -466895527, 2857102551, 8048682737, 1811852719, -36619708247 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..30.

Index entries for linear recurrences with constant coefficients, signature (2,-5).

FORMULA

a(n) = 2*a(n-1) - 5*a(n-2), n>3.

a(n) = left term in [1,-2; 2,1]^n * [1,1].

O.g.f.: -x*(1+5*x)/(1-2*x+5*x^2). a(n)=-A045873(n)-5*A045873(n-1). - R. J. Mathar, Apr 03 2008

a(n) = (1/2)*(1+i)*((1+2*i)^n-i*(1-2*i)^n), where i=sqrt(-1)  - Bruno Berselli, Jul 06 2011

EXAMPLE

a(5) = 79 = 2*a(4) - 5*a(3) = 2*17 - 5*(-9).

a(5) = 79 = left term in [1,-2, 2,1]^5.

PROG

(PARI) a(n)={local(v=Vec((1+2*I*x)^n)); sum(k=1, #v, real(v[k])-imag(v[k])); }

/* cf. A116483 */ /* Joerg Arndt, Jul 06 2011 */

CROSSREFS

Sequence in context: A255830 A256613 A116484 * A320700 A053803 A320323

Adjacent sequences:  A138746 A138747 A138748 * A138750 A138751 A138752

KEYWORD

sign,easy

AUTHOR

Gary W. Adamson, Mar 28 2008

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified June 18 18:52 EDT 2019. Contains 324215 sequences. (Running on oeis4.)