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A169634 a(n) = 3*7^n. 12
3, 21, 147, 1029, 7203, 50421, 352947, 2470629, 17294403, 121060821, 847425747, 5931980229, 41523861603, 290667031221, 2034669218547, 14242684529829, 99698791708803, 697891541961621, 4885240793731347, 34196685556119429 (list; graph; refs; listen; history; text; internal format)
OFFSET
0,1
COMMENTS
Essentially first differences of A120741.
Binomial transform of A169604.
Second binomial transform of A005053 without initial term 1.
Inverse binomial transform of A103333 without initial term 1.
Second inverse binomial transform of A013708.
Except for first term 3, these are the integers that satisfy phi(n) = 4*n/7. - Michel Marcus, Jul 14 2015
Number of distinct quadratic residues (QR) over Z_7^n such that gcd(QR, 7^n) = 1 where n >= 1. - Param Mayurkumar Parekh, Feb 11 2023
LINKS
Richard P. Stanley, Some Linear Recurrences Motivated by Stern's Diatomic Array, arXiv:1901.04647 [math.CO], 2019. Also American Mathematical Monthly 127.2 (2020): 99-111.
FORMULA
a(n) = 7*a(n-1) for n > 0; a(0) = 3.
G.f.: 3/(1-7*x).
PROG
(Magma) [ 3*7^n: n in [0..19] ];
CROSSREFS
Cf. A120741, A169604 (3*6^n), A005053 (expand (1-2x)/(1-5x)), A103333 (expand (1-5x)/(1-8x)), A013708 (3^(2*n+1)), A007283 (3*2^n), A164346 (3*4^n).
Sequence in context: A173350 A323203 A372943 * A088088 A206638 A357652
KEYWORD
nonn,easy
AUTHOR
Klaus Brockhaus, Apr 04 2010
STATUS
approved

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Last modified May 20 13:50 EDT 2024. Contains 372716 sequences. (Running on oeis4.)