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A008845 Numbers k such that k+1 and k/2+1 are squares. 1
0, 48, 1680, 57120, 1940448, 65918160, 2239277040, 76069501248, 2584123765440, 87784138523760, 2982076586042448, 101302819786919520, 3441313796169221280, 116903366249966604048, 3971273138702695316400, 134906383349641674153600, 4582845760749114225906048 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

REFERENCES

A. H. Beiler, Recreations in the Theory of Numbers, Dover, NY, 1964, p. 256.

LINKS

Colin Barker, Table of n, a(n) for n = 0..650

Henry Ernest Dudeney, Amusements in Mathematics, 1917. See problem 114, "Curious numbers".

Index entries for linear recurrences with constant coefficients, signature (35,-35,1).

FORMULA

a(n) = 2*(A008844(n)-1) = 16*A075528(n) = 48*A029546(n). - corrected by Sean A. Irvine, Apr 07 2018

a(0)=0, a(1)=48, a(2)=1680, a(n) = 35*a(n-1)-35*a(n-2)+a(n-3). - Harvey P. Dale, May 24 2014

From Colin Barker, Mar 02 2016: (Start)

a(n) = (-6+(3-2*sqrt(2))*(17+12*sqrt(2))^(-n)+(3+2*sqrt(2))*(17+12*sqrt(2))^n)/4.

G.f.: 48*x / ((1-x)*(1-34*x+x^2)).

(End)

a(n) = 34*a(n-1)-a(n-2)+48. - Vincenzo Librandi, Mar 03 2016

EXAMPLE

48+1 = 49 = 7^2 and 48/2+1 = 24+1 = 25 = 5^2.

MATHEMATICA

LinearRecurrence[{35, -35, 1}, {0, 48, 1680}, 20] (* Harvey P. Dale, May 24 2014 *)

PROG

(PARI) concat(0, Vec(48*x/((1-x)*(1-34*x+x^2)) + O(x^20))) \\ Colin Barker, Mar 02 2016

(MAGMA) I:=[0, 48]; [n le 2 select I[n] else  34*Self(n-1) - Self(n-2)+48: n in [1..20]]; // Vincenzo Librandi, Mar 03 2016

CROSSREFS

Sequence in context: A266160 A004386 A076003 * A273627 A288455 A231450

Adjacent sequences:  A008842 A008843 A008844 * A008846 A008847 A008848

KEYWORD

nonn,easy

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified December 15 13:27 EST 2018. Contains 318149 sequences. (Running on oeis4.)