A110605 Numbers n whose base 7 representations, interpreted as base 10 integers, are semiprimes.

4, 6, 7, 11, 12, 15, 16, 19, 20, 24, 25, 26, 34, 36, 40, 44, 47, 55, 57, 61, 64, 65, 66, 73, 74, 78, 79, 80, 82, 83, 89, 92, 97, 99, 100, 101, 103, 104, 108, 109, 110, 113, 118, 124, 136, 137, 142, 145, 148, 149, 150, 152, 158, 162, 164, 167, 172, 173, 176, 181, 187

Offset

1,1

Comments

A108873 is the equivalent using base 3. A110602 is the equivalent using base 4. A110603 is the equivalent using base 5. A110604 is the equivalent using base 6. A110606 is the equivalent using base 8. A110607 is the equivalent using base 9.

Links

Table of n, a(n) for n=1..61.

Example

a(1) = 4 because 4 (base 7) = 4 (base 10) = 2 * 2, a semiprime (A001358).
a(2) = 6 because 6 (base 7) = 6 (base 10) = 2 * 3
a(3) = 7 because 7 (base 7) = 10 and 10 (base 10) = 2 * 5.
a(4) = 11 because 11 (base 7) = 14 and 14 (base 10) = 2 * 7.
a(5) = 12 because 12 (base 7) = 15 and 15 (base 10) = 3 * 5.

Mathematica

Select[Range[187], Plus @@ Last /@ FactorInteger[FromDigits[IntegerDigits[ #, 7]]] == 2 &] (* Ray Chandler, Aug 05 2005 *)

Crossrefs

Cf. A001358, A007093, A108873, A110602, A110603, A110604, A110606, A110607.

Sequence in context: A039590 A134038 A247546 * A206775 A287157 A102140

Adjacent sequences: A110602 A110603 A110604 * A110606 A110607 A110608

Keyword

base,easy,nonn

Author

Jonathan Vos Post, Jul 30 2005

Extensions

Extended by Ray Chandler, Aug 05 2005

Status

approved