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A241851 a(n) = n^2 + 21. 3
21, 22, 25, 30, 37, 46, 57, 70, 85, 102, 121, 142, 165, 190, 217, 246, 277, 310, 345, 382, 421, 462, 505, 550, 597, 646, 697, 750, 805, 862, 921, 982, 1045, 1110, 1177, 1246, 1317, 1390, 1465, 1542, 1621, 1702, 1785, 1870, 1957, 2046, 2137, 2230, 2325 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,1
LINKS
FORMULA
G.f.: (21 - 41*x + 22*x^2)/(1 - x)^3.
a(n) = a(-n) = 3*a(n-1) - 3*a(n-2) + a(n-3) = a(n-1) + 2*n - 1.
From Amiram Eldar, Nov 04 2020: (Start)
Sum_{n>=0} 1/a(n) = (1 + sqrt(21)*Pi*coth(sqrt(21)*Pi))/42.
Sum_{n>=0} (-1)^n/a(n) = (1 + sqrt(21)*Pi*cosech(sqrt(21)*Pi))/42. (End)
MATHEMATICA
Table[n^2 + 21, {n, 0, 60}]
PROG
(Magma) [n^2+21: n in [0..60]];
(PARI) a(n)=n^2+21 \\ Charles R Greathouse IV, Jun 17 2017
CROSSREFS
Cf. similar sequence listed in A114962.
Sequence in context: A141439 A323419 A333908 * A295747 A125737 A349248
KEYWORD
nonn,easy
AUTHOR
Vincenzo Librandi, May 01 2014
STATUS
approved

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Last modified February 21 12:33 EST 2024. Contains 370235 sequences. (Running on oeis4.)