OFFSET
1,2
COMMENTS
The number 80 is probably the unique non-power of 2 in the sequence.
EXAMPLE
80 is in the sequence because 80 -> 40 -> 20 -> 10 -> 5 -> 16 -> 8 -> 4 -> 2 -> 1 with 9 iterations and the divisors of 80 are 1, 2, 4, 5, 8, 10, 16, 20, 40, 80.
MAPLE
with(numtheory):nn:=10000:
for n from 1 to nn do:
m:=n:it:=0:lst:={n}:
for i from 1 to nn while(m<>1) do:
if irem(m, 2)=0
then
m:=m/2:
else
m:=3*m+1:
fi:
it:=it+1:lst:=lst union {m}:
od:
x:=divisors(n):n0:=nops(x):lst1:={op(x), x[n0]}:
lst2:=lst intersect lst1:n1:=nops(lst2):
if it+1=n1 and n0=n1
then
print(n):
else fi:
od:
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, May 12 2016
STATUS
approved