A245906 - OEIS

A245906

Numbers of the form 4n^2 + 1 or 4n^2 + 8n + 1.

5, 13, 17, 33, 37, 61, 65, 97, 101, 141, 145, 193, 197, 253, 257, 321, 325, 397, 401, 481, 485, 573, 577, 673, 677, 781, 785, 897, 901, 1021, 1025, 1153, 1157, 1293, 1297, 1441, 1445, 1597, 1601, 1761, 1765, 1933, 1937, 2113, 2117, 2301, 2305, 2497, 2501, 2701

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..50.

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

FORMULA

a(n) = n^2 + O(n). - Charles R Greathouse IV, Nov 20 2014

G.f.: x*(5+8*x-6*x^2+x^4)/((1+x)^2*(1-x)^3). [Bruno Berselli, Dec 02 2014]

a(n) = (2*n*(n+5)-(2n+1)*(-1)^n+11)/2. [Bruno Berselli, Dec 02 2014]

MATHEMATICA

fn[n_]:=Module[{c=4n^2+1}, {c, c+8n}]; Flatten[Array[fn, 30]] (* or *) LinearRecurrence[{1, 2, -2, -1, 1}, {5, 13, 17, 33, 37}, 60] (* Harvey P. Dale, May 21 2015 *)

PROG

(PARI) list(lim)=Set(concat(vector(sqrtint((lim-1)\4), n, 4*n^2+1), vector(sqrtint(lim\1+3)\2-1, n, 4*n^2+8*n+1))) \\ Charles R Greathouse IV, Nov 13 2014

(PARI) a(n)=if(n%2, (n+1)^2+1, n^2+4*n+1) \\ Charles R Greathouse IV, Nov 20 2014

(Magma) [IsEven(n) select n^2+4*n+1 else (n+1)^2+1: n in [1..50]]; // Bruno Berselli, Dec 02 2014

CROSSREFS

Sequence in context: A253079 A184851 A211425 * A191108 A216575 A306626

Adjacent sequences: A245903 A245904 A245905 * A245907 A245908 A245909

KEYWORD

nonn,easy

AUTHOR

Jamel Ghanouchi, Nov 13 2014

EXTENSIONS

Extended by Charles R Greathouse IV, Nov 13 2014

STATUS

approved