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

Author

Hugo Pfoertner, Jul 15 2018

Keywords

• nonn
• base
• easy
• fini
• full

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.

Examples

			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.
		

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.

Crossrefs

Cf. A050246.

Programs

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