login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A088411 A088257 indexed by A002110. 0
0, 1, 2, 3, 4, 5, 6, 11, 13, 24, 66, 68, 75, 167, 171, 172, 287, 310, 352, 384, 457, 564, 590, 616, 620, 643, 849 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

LINKS

Table of n, a(n) for n=1..27.

FORMULA

a(n)=k such that A088257(n)=A002110(k).

EXAMPLE

3 is in the sequence because primorial p_3# = 2 * 3 * 5 = 30 has two prime neighbors 29 and 31.

4 is in the sequence because primorial p_4# = 2 * 3 * 5 * 7 = 210 has one prime neighbor 211; 209 = 11 * 19.

7 is not in the sequence because the product of the smallest 7 primes has two composite neighbors.

MAPLE

A:= NULL:

P:= 1: p:= 1;

for n from 1 to 700 do

  p:= nextprime(p);

  P:= P*p;

  if isprime(P+1) or isprime(P-1) then A:= A, n fi

od:

A; # Robert Israel, Aug 03 2016

MATHEMATICA

Select[Range[0, 600], Total@ Boole@ PrimeQ@ {# - 1, # + 1} > 0 &@ Apply[Times, Prime@ Range@ #] &] (* Michael De Vlieger, Aug 03 2016 *)

CROSSREFS

Cf. A002110, A088257.

Sequence in context: A082657 A108378 A084588 * A075073 A157420 A221471

Adjacent sequences:  A088408 A088409 A088410 * A088412 A088413 A088414

KEYWORD

more,nonn

AUTHOR

Ray Chandler, Sep 29 2003

EXTENSIONS

a(22)-a(27) from Michael De Vlieger, Aug 03 2016

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 | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified November 18 11:26 EST 2018. Contains 317302 sequences. (Running on oeis4.)