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A309845 Expansion of e.g.f.: sec(x) + 2*tan(x). 0
1, 2, 1, 4, 5, 32, 61, 544, 1385, 15872, 50521, 707584, 2702765, 44736512, 199360981, 3807514624, 19391512145, 419730685952, 2404879675441, 58177770225664, 370371188237525, 9902996106248192, 69348874393137901, 2030847773013704704, 15514534163557086905, 493842960380415967232 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

Table of n, a(n) for n=0..25.

F. Heneghan and T. K. Petersen, Power series for up-down min-max permutations, 2013.

Masato Kobayashi, A new refinement of Euler numbers on counting alternating permutations, arXiv:1908.00701 [math.CO], 2019.

FORMULA

a(n-2) = |{up-down 2nd-max-lower permutations in S_n}| for n >= 2 (see Definition 3.4 in Kobayashi).

a(n) = A000111(n+2) - A164575(n) (See Definition 3.4 in Kobayashi).

a(n) = A225688(n) + A225689(n) - A164575(n) (See Heneghan-Petersen and Kobayashi articles).

a(2*n) = A000111(2*n) (See Lemma 3.8 in Kobayashi).

a(2*n+1) = 2*A000111(2*n+1) (See Lemma 3.8 in Kobayashi).

MATHEMATICA

CoefficientList[Series[Sec[x]+2Tan[x], {x, 0, 25}], x]*Table[n!, {n, 0, 25}]

PROG

(PARI) my(x='x+O('x^30)); Vec(serlaplace(1/cos(x)+2*tan(x))) \\ Michel Marcus, Aug 20 2019

CROSSREFS

Cf. A000111, A000182, A000364, A164575, A181937, A225688, A225689.

Sequence in context: A243004 A137424 A083007 * A002987 A210958 A188925

Adjacent sequences:  A309842 A309843 A309844 * A309846 A309847 A309848

KEYWORD

nonn,easy

AUTHOR

Stefano Spezia, Aug 20 2019

STATUS

approved

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Last modified February 25 11:54 EST 2020. Contains 332233 sequences. (Running on oeis4.)