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A064268 a(n) = (a(n-1) * a(n-6) + 2 * a(n-3) * a(n-4)) / a(n-7). a(1) = ... = a(7) = 1. Somos-7 variation. 1
1, 1, 1, 1, 1, 1, 1, 3, 5, 7, 13, 43, 113, 521, 1241, 3681, 23657, 177721, 679505, 4674203, 27273277, 275517767, 3496390229, 37043734803, 226196947873, 4391322667601, 81041508965617, 1433151398896001, 25397505914206225, 472652420405241521, 9156799134584424289, 499597377081528480243 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,8

LINKS

Harry J. Smith, Table of n, a(n) for n = 1..100

Index entries for two-way infinite sequences

FORMULA

a(8-n) = a(n).

MAPLE

P:= proc(n) option remember; if n <= 7 then 1 else

(procname(n-1)*procname(n-6)+2*procname(n-3)*procname(n-4))/procname(n-7)

fi; end: seq(P(n), n=0..32); # Paolo P. Lava, Sep 27 2018, after Robert Israel at A018896

MATHEMATICA

RecurrenceTable[{a[1]==a[2]==a[3]==a[4]==a[5]==a[6]==a[7]==1, a[n] == (a[n-1]a[n-6]+2a[n-3]a[n-4])/a[n-7]}, a, {n, 30}] (* Harvey P. Dale, Nov 26 2015 *)

PROG

(PARI) {a(n) = if( n<1, a(8-n), if( n<8, 1, (a(n-1) * a(n-6) + 2 * a(n-3) * a(n-4)) / a(n-7)))}

(PARI) { a7=a6=a5=a4=a3=a2=a1=a=1; for (n=1, 100, if (n>7, a=(a1*a6 + 2*a3*a4)/a7; a7=a6; a6=a5; a5=a4; a4=a3; a3=a2; a2=a1; a1=a); write("b064268.txt", n, " ", a) ) } \\ Harry J. Smith, Sep 10 2009

(MAGMA) I:=[1, 1, 1, 1, 1, 1, 1]; [n le 7 select I[n] else (Self(n-1)*Self(n-6) + 2*Self(n-3)*Self(n-4))/Self(n-7): n in [1..30]]; // G. C. Greubel, Feb 21 2018

CROSSREFS

Cf. A006723.

Sequence in context: A163080 A141414 A236464 * A235873 A118743 A225226

Adjacent sequences:  A064265 A064266 A064267 * A064269 A064270 A064271

KEYWORD

nonn,easy

AUTHOR

Michael Somos, Sep 24 2001

STATUS

approved

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Last modified August 13 05:41 EDT 2020. Contains 336442 sequences. (Running on oeis4.)