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A088411 A088257 indexed by A002110. 1
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, 1391, 1552, 1613, 1849, 2122, 2647, 2673, 4413, 13494, 31260, 33237, 67132, 85586, 234725 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,3
COMMENTS
Union of A057704 and A014545. - Jeppe Stig Nielsen, Aug 01 2019
LINKS
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 *)
PROG
(PARI) is(k)=pr=prod(j=1, k, prime(j)); ispseudoprime(pr-1)||ispseudoprime(pr+1) \\ Jeppe Stig Nielsen, Aug 01 2019
CROSSREFS
Sequence in context: A108378 A084588 A309548 * A075073 A157420 A221471
KEYWORD
nonn,more
AUTHOR
Ray Chandler, Sep 29 2003
EXTENSIONS
a(22)-a(27) from Michael De Vlieger, Aug 03 2016
a(28)-a(40) from Jeppe Stig Nielsen, Aug 01 2019
a(41) from Jeppe Stig Nielsen, Oct 19 2021
STATUS
approved

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Last modified June 24 01:53 EDT 2024. Contains 373661 sequences. (Running on oeis4.)