OFFSET
1,1
COMMENTS
Since all consecutive primes, p < q and p greater than 2, are odd, therefore gcd( p-1, q-1 ) must be even.
EXAMPLE
a(49) = 604073 because gcd(604073-1, 604171-1) = gcd(6164*98, 6165*98) = 98 = 2n.
a(4) = 89 because gcd(89-1, 97-1) = gcd(8*11, 8*16) = 8 = 2n and these primes are the smallest with this property.
MATHEMATICA
a = Table[0, {100}]; p = 3; q = 5; Do[q = Prime[n + 1]; d = GCD[p - 1, q - 1]/2; If[d < 101 && a[[d]] == 0, a[[d]] = n]; b = c, {n, 2, 10^7}]; a
With[{tsp={#[[1]], #[[2]], GCD[#[[1]]-1, #[[2]]-1]}&/@Partition[Prime[ Range[ 300000]], 2, 1]}, Transpose[Flatten[Table[Select[tsp, Last[#]==2n&, 1], {n, 40}], 1]][[1]]] (* Harvey P. Dale, Jul 07 2013 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Dec 06 2000
EXTENSIONS
Edited by Robert G. Wilson v, Feb 01 2002
STATUS
approved