login

Reminder: The OEIS is hiring a new managing editor, and the application deadline is January 26.

Smaller of two consecutive primes p and q such that c(p) = c(q), where c(n) = A008908(n) is the length of x, f(x), f(f(x)), ... , 1 in the Collatz conjecture.
1

%I #19 Sep 12 2016 17:00:46

%S 173,409,419,421,439,487,521,557,571,617,761,887,919,1009,1039,1117,

%T 1153,1171,1217,1327,1373,1549,1559,1571,1657,1693,1709,1721,1733,

%U 1783,1831,1861,1901,1993,1997,2053,2089,2339,2393,2521,2539,2647,2657,2677,2693,2777

%N Smaller of two consecutive primes p and q such that c(p) = c(q), where c(n) = A008908(n) is the length of x, f(x), f(f(x)), ... , 1 in the Collatz conjecture.

%C If x is even f(x) = x/2 else f(x) = 3x + 1.

%H Abhiram R Devesh, <a href="/A275968/b275968.txt">Table of n, a(n) for n = 1..10000</a>

%e a(1) = p = 173; q = 179

%e c(p) = c(q) = 32

%t t = Table[Length@ NestWhileList[If[EvenQ@ #, #/2, 3 # + 1] &, n, # != 1 &] - 1, {n, 10^4}]; Prime@ Flatten@ Position[#, k_ /; Length@ k == 1] &@ Map[Union@ Part[t, #] &, #] &@ Partition[#, 2, 1] &@ Prime@ Range@ 410 (* _Michael De Vlieger_, Sep 01 2016 *)

%o (PARI) A008908(n)=my(c=1); while(n>1, n=if(n%2, 3*n+1, n/2); c++); c

%o t=A008908(p=2); forprime(q=3,1e4, tt=A008908(q); if(t==tt, print1(p", ")); p=q; t=tt) \\ _Charles R Greathouse IV_, Sep 01 2016

%o (Python)

%o import sympy

%o def lcs(n):

%o ....a=1

%o ....while n>1:

%o ........if n%2==0:

%o ............n=n//2

%o ........else:

%o ............n=(3*n)+1

%o ........a=a+1

%o ....return(a)

%o m=2

%o while m>0:

%o ....n=sympy.nextprime(m)

%o ....if lcs(m)==lcs(n):

%o ........print(m,)

%o ....m=n

%o # _Abhiram R Devesh_, Sep 02 2016

%Y Cf. A006577 (Collatz trajectory lengths), A078417, A008908.

%K nonn,easy

%O 1,1

%A _Abhiram R Devesh_, Aug 15 2016