

A090805


A simple recurrence with one error.


3



1, 2, 6, 21, 85, 430, 2586, 18109, 144880, 1303929, 13039300, 143432311, 1721187744, 22375440685, 313256169604, 4698842544075, 75181480705216, 1278085171988689, 23005533095796420, 437105128820131999, 8742102576402640000, 183584154104455440021, 4038851390298019680484
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OFFSET

0,2


COMMENTS

I included this in the OEIS only because was published on a web page. The explanation is my own  perhaps the original proposer had a different explanation.


REFERENCES

Found on a puzzle page.


LINKS

Table of n, a(n) for n=0..22.
Hugo Delestinne, Meerdaelquiz
Neil Sloane and Brady Haran, A Sequence with a Mistake, Numberphile video (2021)


FORMULA

a(0) = 1; a(n) = n*(a(n1) + 1) but make an error if n = 4.
Hans Havermann points out that the first 7 terms could also be produced by the recurrence f[x] = f[x  1]*(x  1) + GCD[3*f[x  1], (x  1)] with f[1] = 1. (This gives the continuation 1, 2, 6, 21, 85, 430, 2586, 18103, 144825, 1303434, 13034342, ...) But given the nature of the other problems on this quiz, I think my explanation is more likely.


EXAMPLE

1..add.1..multiply.by 1 > 2
2..add.1..multiply.by 2 > 6
6......1............. 3 > 21
21.....1............. 4 > 88 but here you make a mistake and instead multiply by 4 and add 1, getting 85
85.....1............. 5 > 430
430....1............. 6 > 2586
etc


MAPLE

a:= proc(n) a(n):= n*a(n1) + `if`(n=4, 1, n) end: a(0):= 1:
seq(a(n), n=0..22); # Alois P. Heinz, May 14 2021


CROSSREFS

Cf. A033540, A344229.
Sequence in context: A099947 A121726 A344229 * A150226 A326335 A256180
Adjacent sequences: A090802 A090803 A090804 * A090806 A090807 A090808


KEYWORD

nonn,easy,dumb


AUTHOR

N. J. A. Sloane, Feb 12 2004


STATUS

approved



