A061310 - OEIS

A061310

Numbers which cannot be reached in the Countdown Numbers Game starting from (1,2,3,4,5,6): the game allows brackets and the operations + - x and / and not all numbers need be used, while the result of each partial calculation must be an integer.

%I #18 Feb 08 2023 10:59:17

%S 284,307,309,314,317,379,394,398,403,411,417,423,429,431,433,435,436,

%T 437,439,441,442,443,445,446,447,449,451,453,454,457,458,459,461,463,

%U 464,466,467,469,471,473,474,487,489,491,493,494,496,499,502,506,509

%N Numbers which cannot be reached in the Countdown Numbers Game starting from (1,2,3,4,5,6): the game allows brackets and the operations + - x and / and not all numbers need be used, while the result of each partial calculation must be an integer.

%C Starting with (1,1,1,1,1,1) no number greater than 9 can be reached.

%C Every number greater than 1080 = 6*5*4*3*(2+1) is in the sequence. - _Sean A. Irvine_, Feb 07 2023

%H Michael S. Branicky, <a href="/A061310/b061310.txt">Table of n, a(n) for n = 1..538</a> (all terms < 1081)

%H Henry Bottomley, <a href="http://www.se16.info/js/cdown.htm">Countdown Numbers Game applet</a>

%H Michael S. Branicky, <a href="/A061310/a061310.py.txt">Python program</a>

%H <a href="/index/Fo#4x4">Index entries for similar sequences</a>

%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1).

%e 283 is not in the sequence since 283=((6+1)*5*4*2)+3; 285 is not in the sequence since 285=((6*5*2)-3)*(4+1). However there is no similar calculation for 284 so 284 is in the sequence.

%K nonn,easy

%O 1,1

%A _Henry Bottomley_, Jun 05 2001