A316603 Double prime times.

13, 17, 23, 29, 31, 37, 59, 101, 103, 107, 127, 223, 227, 229, 233, 239, 251, 311, 331, 349, 353, 359, 401, 409, 419, 421, 457, 509, 521, 523, 541, 547, 601, 631, 647, 701, 733, 751, 811, 827, 829, 839, 853, 911, 929, 937, 941, 953, 1013, 1021, 1039, 1051, 1109, 1151, 1213

Offset

1,1

Comments

Numbers on the display of a 4-digit hh:mm digital clock that remain prime when the display mode is switched between 12-hour AM/PM and 24-hour time.

Links

Hugo Pfoertner, Table of n, a(n) for n = 1..108

Forums for the webcomic xkcd.com, 0247: "Factoring the Time", post by user jasticE, Apr 12 2007.

Randall Munroe, Factoring the time, xkcd Web Comic #247, Apr 11 2007.

Example

a(49) = 1013 and a(103) = 2213 are in the sequence, because toggling the display mode of the digital clock will either leave 10:13 unchanged or switch between 22:13 and 10:13 at 10:13 PM. Both displayed times are prime when reading hh:mm as decimal number.

Prog

(PARI) apd(x)=2400*((x>1200)-1/2);
for(h=0, 23, for(m=0, 59, t=100*h+m; t12=t-apd(t); if(isprime(t)&&isprime(t12), print1(t, ", ")))) \\ Hugo Pfoertner, Jul 18 2018

Crossrefs

Cf. A050246.

Sequence in context: A080603 A179924 A274321 * A235914 A050716 A217046

Adjacent sequences: A316600 A316601 A316602 * A316604 A316605 A316606

Keyword

nonn,base,easy,fini,full

Author

Hugo Pfoertner, Jul 15 2018

Status

approved