1,1

Paolo P. Lava, Table of n, a(n) for n = 1..1000

3 is in the sequence because 3 is prime and both 2*3 - 1 = 5 and 2*3 + 1 = 7 are squarefree.

with(numtheory); P:=proc(q) local n; for n from 1 to q do

if isprime(n) then if issqrfree(2*n-1) and issqrfree(2*n+1) then print(n)

fi; fi; od; end: P(10^5); #Paolo P. Lava, Jun 26 2013

(PARI) is(n)=isprime(n)&&issquarefree(2*n-1)&&issquarefree(2*n+1) \\ Charles R Greathouse IV, Jun 27 2013

Cf. A005117.

Sequence in context: A040103 A045323 A161185 * A235482 A155141 A127944

Adjacent sequences: A226990 A226991 A226992 * A226994 A226995 A226996

nonn

Irina Gerasimova, Jun 26 2013

approved