|
|
A078770
|
|
a(n) = the least positive integer k such that k^2 + k + N is prime, where N is the n-th positive odd integer.
|
|
1
|
|
|
1, 1, 1, 2, 1, 1, 2, 1, 1, 3, 1, 2, 2, 1, 1, 2, 4, 1, 2, 1, 1, 5, 1, 2, 3, 1, 2, 2, 1, 1, 2, 4, 1, 2, 1, 1, 2, 7, 1, 5, 1, 2, 3, 1, 3, 2, 4, 1, 2, 1, 1, 2, 1, 1, 5, 1, 10, 3, 4, 3, 2, 7, 1, 3, 1, 2, 2, 1, 1, 3, 7, 2, 2, 1, 1, 2, 4, 1, 2, 4, 1, 5, 1, 2, 3, 1
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,4
|
|
COMMENTS
|
k^2 + k + n for even n is always even and > 2, so is never prime.
|
|
LINKS
|
|
|
EXAMPLE
|
For n=1, k^2+k+1 is prime for k=1, since it is 3.
For n=7, k^2+k+7 is not prime for k=1, but is prime for k=2, since it is 13.
|
|
MATHEMATICA
|
lpik[n_]:=Module[{k=1}, While[!PrimeQ[k^2+k+n], k++]; k]; Table[lpik[n], {n, 1, 181, 2}] (* Harvey P. Dale, Sep 24 2017 *)
|
|
PROG
|
(PARI) lista(nn) = {forstep (n=1, nn, 2, k = 1; while(! isprime(k*k + k + n), k++); print1(k, ", "); ); } \\ Michel Marcus, May 18 2013
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|