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Number of operations A000142 (i.e., x!) or A000196 (i.e., floor(sqrt(x))) needed to get n, starting with 3.
2

%I #23 Sep 20 2013 05:31:01

%S 1,2,0,20,4,1,14,17,31,6,26,41,35,20,31,31,19,28,27,38,21,33,21,21,26,

%T 3,51,38,28,26,20,35,36,36,13,23,27,62,45,50,45,40,9,15,31,8,32,52,36,

%U 13,68,69,57,33,54,36,46,34,49,63,56,68,14,63,23,33,36,47,43,16,38,66,38

%N Number of operations A000142 (i.e., x!) or A000196 (i.e., floor(sqrt(x))) needed to get n, starting with 3.

%C Knuth conjectured that any number can be obtained in that way, starting from 4.

%C This seems also to be true using 3 as the starting value. Since 3 is the minimal possible choice, this variant could be considered to be more natural.

%C To ensure the sequence is well-defined, define a(n)=-1 if it is not possible to get n in the given way.

%C See A139004 for references and links.

%H Jon E. Schoenfield, <a href="/A139003/b139003.txt">Table of n, a(n) for n = 1..1000</a>

%H Jon E. Schoenfield, <a href="/A139003/a139003.txt">Table of n, a(n), and shortest path for n = 1..1000</a>

%F a(3) = 0; a(n) = min { a(k)+1 ; n^2 <= k < (n+1)^2 or k! = n }

%e Representing the operation x -> floor(sqrt(x)) by "s" and x -> x! by "f",

%e we have:

%e a(1) = 1 since 1 = s3 is clearly the shortest way to obtain 1 from 3.

%e a(2) = 2 since 2 = sf3 is clearly the shortest way to obtain 2 from 3.

%e a(3) = 0 since no operation is required to get 3 which is there at the beginning.

%e a(5) = 4 since 5 = ssff3 is the shortest way to obtain 5 from 3.

%e a(6) = 1 since 6 = f3 is certainly the shortest way to get 6 from 3.

%e a(4) = 20 = 7+9+a(5) since 4 = ssssssfsssssssffssff3 = floor(35!^(1/2^6)), 35 = floor((5!)!^(1/2^7)).

%o (PARI) A139003( n, S=Set(3), LIM=10^5 )={ for( i=0,LIM, setsearch( S, n) & return(i); S=setunion( S, setunion( Set( vector( #S, j, sqrtint(eval(S[j])))), Set( vector( #S, j, if( LIM > j=eval(S[j]), j!))))))}

%Y Cf. A139004.

%K nonn

%O 1,2

%A _M. F. Hasler_, Apr 09 2008

%E a(9)-a(11) from _Max Alekseyev_, Nov 03 2008

%E Corrected formula, added terms from a(12) onward. - _Jon E. Schoenfield_, Nov 17 2008, Nov 19 2008

%E Comments and example edited by _Jon E. Schoenfield_, Sep 15 2013