A080864 Numbers n such that spf(n)+2 = spf(n-2), where spf=A020639 (smallest prime factor).

27, 57, 87, 117, 147, 177, 205, 207, 237, 267, 297, 327, 357, 387, 415, 417, 447, 477, 507, 537, 567, 597, 625, 627, 657, 687, 717, 747, 777, 807, 835, 837, 867, 897, 927, 957, 987, 1017, 1045, 1047, 1077, 1107, 1137, 1167, 1197, 1227, 1255, 1257, 1287

Offset

1,1

Comments

(spf(n),spf(n-2)) are twin prime pairs.

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Example

1921=17*113, 1921-2=1919=19*101 and (17,19)=(A001359(4), A006512(4)), therefore 1921 is a term: a(74)=1921, A080863(74)=1919.

Mathematica

Flatten[Position[Partition[Array[FactorInteger[#][[1, 1]]&, 1300], 3, 1], _?(#[[1]]==#[[3]]+2&), {1}, Heads->False]]+2 (* Harvey P. Dale, Oct 15 2015 *)

Crossrefs

Cf. A080863(n)=a(n)-2, A001359, A006512.

Sequence in context: A132768 A352266 A053180 * A260440 A039459 A235164

Adjacent sequences: A080861 A080862 A080863 * A080865 A080866 A080867

Keyword

nonn

Author

Reinhard Zumkeller, Feb 21 2003

Status

approved