OFFSET
1,1
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1001 from Harvey P. Dale)
EXAMPLE
71 is a term because it is a prime, 71 + 2 = 73 is a prime, and 71 - 1 = 70 = 2 * 5 * 7 is squarefree.
17 is not a term because 17 - 1 = 2^4.
MATHEMATICA
Select[Transpose[Select[Partition[Prime[Range[600]], 2, 1], #[[2]]-#[[1]]==2&]][[1]], SquareFreeQ[#-1]&] (* Harvey P. Dale, Aug 10 2013 *)
PROG
(PARI) pm1th(n) = { c=0; pc=0; forprime(x=2, n, pc++; y=x-1; if(isprime(x+2), if(issquarefree(y), c++; print1(x", "); ) ) ); print(); print(c", "pc", "c/pc+.0) }
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Cino Hilliard, Dec 07 2003
EXTENSIONS
Offset corrected by Amiram Eldar, Jun 29 2024
STATUS
approved