OFFSET
1,3
COMMENTS
The Collatz trajectory of the number n is a sequence starting with n and ending with 1 (obtained for the first time), constructed using even steps k to k/2 and odd steps k to 3*k+1. a(n) is the number of times in the Collatz trajectory of n that the number 3*k+1, obtained after the odd step k to 3*k+1, is greater than n. Alternatively, a(n) represents the number of odd terms in the Collatz trajectory of n that are greater than (n-1)/3. a(1)=0 since the Collatz trajectory of 1 has no steps.
LINKS
Michael De Vlieger, Table of n, a(n) for n = 1..10000
FORMULA
a(n) <= A006667(n). - David A. Corneth, May 20 2020
EXAMPLE
The Collatz trajectory of n=3 is 3, (10), 5, (16), 8, 4, 2, 1. It happens twice that the number 3*k+1 in this process is greater than n (those numbers 3*k+1 are in parentheses), so a(3)=2.
MATHEMATICA
a[n_] := Block[{c = NestWhileList[ If[ EvenQ@ #, #/2, 3 # + 1] &, n, #>1 & ]}, Length@ Select[ Range[2, Length[c]], OddQ[c[[# - 1]]] && c[[#]] > n &]]; Array[a, 90] (* Giovanni Resta, May 19 2020 *)
PROG
(PARI) a(n) = my(res=0, cn=n); while(n>1, if(bitand(n, 1), n=3*n+1; if(n>cn, res++); , n>>=1)); res \\ David A. Corneth, May 20 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Hamid Kulosman, May 11 2020
EXTENSIONS
More terms from Giovanni Resta, May 19 2020
STATUS
approved