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 A213215 For the Collatz (3x+1) iterations starting with the odd numbers k, a(n) is the smallest k such that the trajectory contains n successive odd numbers == 3 (mod 4). 1
 3, 7, 15, 27, 27, 127, 255, 511, 1023, 1819, 4095, 4255, 16383, 32767, 65535, 77671, 262143, 459759, 1048575 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS n is possibly counted. The sequence is infinite because the Collatz trajectory starting at k = 2^n - 1 contains at least n consecutive odd numbers == 3(mod 4) such that 3*2^n - 1 -> 3^2*2^(n-1)-1 -> ... -> 2*3^(n-1)-1 and then -> 3^n-1 -> ...  but the numbers of this sequence are not always of this form, for example 27, 1819, 4255, 77671, 459759, ... LINKS EXAMPLE a(3)=15 because the Collatz iterations starting at 15 and 6 are : 15, 46, 23, 70, 35, 106, 53, 160, 80, 40, 20, 10, 5, 16, 8, 4, 2, 1 and the 3 consecutive odd numbers 15, 23 and 35 are congruent to 3 (mod 4). MAPLE nn:=200:T:=array(1..nn): for n from 1 to 20 do:jj:=0:          for m from 3 by 2 to 10^8 while(jj=0) do:                for i from 1 to nn while(jj=0) do:                T[i]:=0:od:a:=1:T:=m:x:=m:                      for it from 1 to 100 while (x>1) do:                          if irem(x, 2)=0 then                          x := x/2:a:=a+1:T[a]:=x:                          else                          x := 3*x+1: a := a+1: T[a]:=x:                         fi:                      od:                      jj:=0:aa:=a:                        for j from 1 to aa while(jj=0) do:                          if irem(T[j], 4)=3 then                          T[j]:=1:                          else                          T[j]:=0:                        fi:                       od:                          for p from 0 to aa-1 while (jj=0) do:                          s:=sum(T[p+k], k=1..2*n):                          if s=n then                          jj:=1: printf ( "%d %d \n", n, m):                          else                          fi:                   od:               od:            od: MATHEMATICA Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; countThrees[t_] := Module[{mx = 0, cnt = 0, i = 0}, While[i < Length[t], i++; If[t[[i]] == 3, cnt++; i++, If[cnt > mx, mx = cnt]; cnt = 0]]; mx]; nn = 15; t = Table[0, {nn}]; n = 1; While[Min[t] == 0, n = n + 2; c = countThrees[Mod[Collatz[n], 4]]; If[c <= nn && t[[c]] == 0, t[[c]] = n; Do[If[t[[i]] == 0, t[[i]] = n], {i, c}]]]; t (* T. D. Noe, Mar 02 2013 *) CROSSREFS Cf. A006577, A000225, A024023, A213214. Cf. A222598 (similar). Sequence in context: A001649 A303220 A301894 * A324719 A170884 A182836 Adjacent sequences:  A213212 A213213 A213214 * A213216 A213217 A213218 KEYWORD nonn AUTHOR Michel Lagneau, Mar 02 2013 STATUS approved

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Last modified May 9 13:09 EDT 2021. Contains 343742 sequences. (Running on oeis4.)