

A316603


Double prime times.


2



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
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OFFSET

1,1


COMMENTS

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



