Problems & Puzzles:
& its digital roots
Here I'm asking by runs of K consecutive
primes having the same digital root. Example. For K=7, all these consecutive
primes (18410243 18410261 18410279 18410333 18410351 18410369, 18410387) have the same
digital root (5).
Q1. Send your a
Regarding the following recursive
formula: p+dr(p) = q; q+dr(q) = r,... where p, q, r, ... are primes,
Q2. How far and why
that formula can go?
Contributions came from J. K. Andersen, Farideh
Firoozbakht, Torbjörn Alm,
Jan van Delden.
J. K. Andersen wrote:
Q1. The digital root is the value modulo 9. The longest
run at (2006)
is 12 primes with digital root 7, starting at 4016465016163.
Q2. The sequence of digital roots is determined by the first number.
A prime sequence cannot continue after the first odd digital root, because
the next number will be even. The longest possible sequence is four primes
with digital roots 2, 4, 8, 7. The first case is 227, 229, 233, 241. The
first with consecutive primes is 2657, 2659, 2663, 2671.
Farideh Firoozbakht wrote:
All these 8 consecutive primes 716230591,716230609,716230699,716230717,
716230771,716230807,716230843 & 716230861 have the same
digital root (7).
Torbjörn Alm wrote:
Searching up to 2000000000 I found 5 chains of size 8
besides numerous chains of size 7 etc.
This is the largest one:
Run length: 8 primitive root= 4
1324001281 1324001299 1324001317 1324001353 1324001389 1324001407
Hakan Summakoğlu wrote
For K=8, all these consecutive primes
(716230591 , 716230609 , 716230699 , 716230717,
716230771 , 716230807 , 716230843 , 716230861) have the same digital root
Emmanuel Vanieghem wrote:
My longest run is 10930792111, ... ,10930792399 (9
consecutive primes with digital root 7).
About Q2, the longest sequence involves four primes at
Proof : if p is prime and p+dr(p) too, then dr(p) =
2, 4 or 8.
If dr(p) = 8, then dr(p+dr(p)) = dr(16) = 7 and
q+dr(q) cannot be prime.
If dr(p) = 4, then dr(p+dr(p)) = 8, whence r =
q+dr(q) can be prime, but s = r+dr(r) not.
If dr(p) = 2, then dr(p+dr(p)) = 4, whence q, r,
and s can be prime, but s+dr(s) not.
The situation of four primes occurs first for p = 227
(the others being 229, 233 and 241).
Jan van Delden wrote:
Q1: The primes 716230591..716230861 give a run of 8
consecutive primes with digital root 7.
Q2: The maximal length is 4, with p equal
to 47 mod 90. [In theory, I didn’t find one yet, length 3: [11,13,17]].
For a prime p, the digital root dr(p) is
equal to 1,2,4,5,7 or 8 and dr(p)=p mod 9. [Except for 3 where dr(3)=3]
Since we want q=p+dr(p) prime, we should have dr(p) equal to either 2,4
or 8, so p is either 2,4 or 8 mod 9.
If p=2 mod 9, q=p+dr(p)=4 mod 9, r=q+dr(q)=8
mod 9 and s=r+dr(r)=7 mod 9 continuing we get 5,1,2 and we get a cyclic
group of moduli.
However 2,4,8, etc. are small gaps so we could investigate the last
digit of the primes involved. Modulo 10 our p should be either 1,3,7 or
If p=2 mod 9 with the last digit equal to
7 we get a length of 4 with ending digits: [7,9,3,1] and the next term
would end in 8. Starting p with a different last digit would end sooner
with last digit 5. Starting with p=4 (or p=8 mod 9) would make the
sequence shorter because the last digit would become even 1 (or 2)
Our “champion” p should therefore be 47
mod 90. Giving primes of the form [47,49,53,61] mod 90. I searched until
10^10 and didn’t find a solution.
For “large” p it will be hard to find such
a solution because we need a sequence with small gaps [2,4,8].
Later Jan accepted that he had a bug in his
code that skipped the solution
227, 229, 233, 241 & others.