

A187815


Number of permutations q_1, ..., q_7 of the 7 consecutive primes p_n, p_{n+1}, ..., p_{n+6} with q_1 = p_n and q_7 = p_{n+6}, and with q_1q_2, q_2q_3, ..., q_6q_7, q_7q_1 pairwise distinct, where p_k denotes the kth prime.


3



10, 2, 7, 4, 10, 17, 15, 15, 17, 11, 4, 23, 33, 24, 19, 16, 24, 16, 31, 39, 39, 30, 24, 11, 15, 39, 30, 52, 66, 41, 29, 23, 48, 43, 15, 15, 43, 48, 39, 30, 30, 52, 68, 64, 68, 34, 19, 27, 39, 35, 22, 36, 32, 20, 19, 32, 38, 72, 71, 59
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OFFSET

1,1


COMMENTS

For each k = 3,4,5,6 there are k consecutive primes p_n, p_{n+1}, ..., p_{n+k1} such that there is no permutation q_1, ..., q_k of p_n, p_{n+1}, ..., p_{n+k1} with q_1q_2, ..., q_{k1}q_k, q_kq_1 pairwise distinct. Such consecutive primes include (3, 5, 7), (5, 7, 11, 13), (3, 5, 7, 11, 13), and (p_{2209}, p_{2210}, ..., p_{2214}) = (19471, 19477, 19483, 19489, 19501, 19507).
For k > 7 the author once thought that for any k consecutive primes p_n, p_{n+1}, ..., p_{n+k1} there always exists a permutation q_1, ..., q_k of p_n, p_{n+1}, ..., p_{n+k1} with q_1q_2, ..., q_{k1}q_k, q_kq_1 pairwise distinct. But this is unlikely to be true as pointed out by Noam D. Elkies.
See also A185645 for a related conjecture.


LINKS

ZhiWei Sun, Table of n, a(n) for n = 1..10000
Noam D. Elkies, Re: A conjecture on permutations of consecutive primes, a message to Number Theory List, August 31, 2013.


EXAMPLE

a(2) = 2 since there are exactly two permutations q_1,...,q_7 of 3,5,7,11,13,17,19 meeting the requirement: (q_1,...,q_7) = (3, 7, 17, 11, 13, 5, 19), (3, 11, 13, 7, 17, 5, 19).


MATHEMATICA

V[n_, i_]:=Part[Permutations[{Prime[n+1], Prime[n+2], Prime[n+3], Prime[n+4], Prime[n+5]}], i]
Do[m=0; Do[If[Length[Union[{Abs[Part[V[n, i], 1]Prime[n]]}, Table[Abs[Part[V[n, i], j]If[j<5, Part[V[n, i], j+1], Prime[n+6]]], {j, 1, 5}]]]<6, Goto[aa]];
m=m+1; Label[aa]; Continue, {i, 1, 5!}]; Print[n, " ", m]; Continue, {n, 1, 20}]
A187815[n_] := Module[{p, c = 0, i = 1, j, q},
p = Permutations[Table[Prime[j], {j, n + 1, n + 5}]];
While[i <= Length[p],
q = Join[{Prime[n]}, p[[i]], {Prime[n + 6]}]; i++;
If[Length[
Union[Join[
Table[Abs[q[[j]]  q[[j + 1]]], {j, 1, 6}], {Abs[
q[[7]]  q[[1]]]}]]] == 7, c++]]; c];
Table[A187815[n], {n, 1, 60}] (* Robert Price, Apr 04 2019 *)


CROSSREFS

Cf. A000040, A185645, A228728.
Sequence in context: A037922 A111287 A255668 * A318486 A303850 A303848
Adjacent sequences: A187812 A187813 A187814 * A187816 A187817 A187818


KEYWORD

nonn


AUTHOR

ZhiWei Sun, Aug 30 2013


STATUS

approved



