|
|
A178984
|
|
a(n) is the smallest prime p that makes prime(n) + 1 - p a prime.
|
|
0
|
|
|
3, 3, 5, 3, 5, 3, 5, 7, 3, 7, 5, 3, 5, 7, 7, 3, 7, 5, 3, 7, 5, 7, 19, 5, 3, 5, 3, 5, 19, 5, 7, 3, 11, 3, 7, 7, 5, 7, 7, 3, 11, 3, 5, 3, 13, 13, 5, 3, 5, 7, 3, 11, 7, 7, 7, 3, 7, 5, 3, 11, 31, 5, 3, 5, 19, 7, 11, 3, 5, 7, 19, 7, 7, 5, 7, 19, 5, 13, 11, 3, 11, 3
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
3,1
|
|
COMMENTS
|
If Goldbach's conjecture is true, this sequence is defined for all n.
|
|
LINKS
|
|
|
EXAMPLE
|
For n = 3, prime(3) = 5, and a(3) = 3 because 5 + 1 - 3 = 3 is prime;
For n = 4, prime(4) = 7, and a(4) = 3 because 7 + 1 - 3 = 5 is prime;
...
for n = 25, prime(n) = 97, and a(n) = 19 because 97 + 1 - 19 = 79 is prime.
|
|
MATHEMATICA
|
f[n_] := Block[{i = 2, p = Prime[n + 2]},
While[q = Prime[i]; ! PrimeQ[p + 1 - q], i++]; q]; Array[f, 60]
|
|
PROG
|
(PARI) a(n) = my(p=prime(n), q=2); while (!isprime(p+1-q), q = nextprime(q+1)); q; \\ Michel Marcus, Apr 01 2020
|
|
CROSSREFS
|
Differs from A119912 after 23 terms.
|
|
KEYWORD
|
nonn,easy
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|