login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A064821 Number of ways of writing the numbers 1 .. n in a sequence so that the sum of any two adjacent numbers is a prime; reversing the sequence does not count as different. 3
0, 1, 1, 4, 2, 8, 12, 30, 70, 664, 1072, 8768, 11648, 37108, 95772, 1059816, 2047488, 12111712, 22802028, 120779959, 337801784, 4361743860, 11425028900, 142573286216, 429917269469, 4138239848098, 16171519847028, 214845911686065 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

If the sequence is d_1 d_2 ... d_n then the n-1 sums d_i + d_{i+1} are required to be primes.

I conjecture a(n) > 0 for all n.

Variant of A051239 with respect to a(1). - R. J. Mathar, Oct 02 2008

LINKS

Table of n, a(n) for n=1..28.

EXAMPLE

For n = 4 there are 4 sequences: 1234, 1432, 3214, 3412.

CROSSREFS

Cf. A064821, A103839.

Sequence in context: A246202 A134042 A051239 * A002291 A225872 A231777

Adjacent sequences:  A064818 A064819 A064820 * A064822 A064823 A064824

KEYWORD

nonn,nice

AUTHOR

N. J. A. Sloane, Oct 23 2001

EXTENSIONS

More terms from Jud McCranie, Oct 24 2001

a(22)-a(24) from Donovan Johnson, Aug 27 2010

a(25)-a(28) from Giovanni Resta, Jun 05 2020

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified August 2 18:52 EDT 2021. Contains 346428 sequences. (Running on oeis4.)