A129336 Digital clock semiprimes.

4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38, 39, 46, 49, 51, 55, 57, 58, 106, 111, 115, 118, 119, 121, 122, 123, 129, 133, 134, 141, 142, 143, 145, 146, 155, 158, 159, 201, 202, 203, 205, 206, 209, 213, 214, 215, 217, 218, 219, 221, 226, 235, 237, 247

Offset

1,1

Comments

Semiprime analog of A050246. Semiprimes possible on a 24-hour digital clock, with no seconds. The largest value is a(414) = 2359 = 7 * 337 because 23:59 is the largest 4-digit number that appears on a 24-hour digital clock.

Links

Nathaniel Johnston, Table of n, a(n) for n = 1..414 (full sequence)

Randall Munroe, Factoring the Time

Eric Weisstein's World of Mathematics, Clock Prime

Example

253 is in the sequence because (see the comic) a character looks at a digital clock reading 2:53 and says: "253 is 11 x 23." That clock-time is a semiprime.

Maple

with(numtheory): for h from 0 to 23 do for m from 0 to 59 do t:=100*h+m: if(bigomega(t)=2)then printf("%d, ", t): fi: od: od: # Nathaniel Johnston, May 17 2011

Crossrefs

Cf. A001358, A050246, A118848, A118849, A118850.

Sequence in context: A176540 A108764 A193801 * A226526 A103607 A264815

Adjacent sequences: A129333 A129334 A129335 * A129337 A129338 A129339

Keyword

base,easy,fini,full,nonn

Author

Jonathan Vos Post, May 27 2007

Status

approved