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Right Prime

Go to solution Solved by Molly Mae,

Question

Say 5939 is a "right" prime because it remains prime after dropping any number of digits from the right: 5939, 593, 59, and 5 are all prime. How many right primes are there less than 1000?

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• Solution

Some amount of primes.

27

2, 3, 5, 7, 23, 29, 31, 37, 53, 59, 71, 73, 79, 233, 239, 293, 311, 313, 317, 373, 379, 593, 599, 719, 733, 739, 797

So I started with single digit prime numbers, then I just built a tree for two-digit primes that begin with one of those one-digit primes.  I did the same thing for the three-digit primes that begin with one of the two-digit primes.

Edited by Molly Mae
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Any other prime numbers less than 1000.

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Spoiler

23 "right" primes if 2, 3, 5 and 7 (nothing to drop on the right) are not considered.

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FWIW

Spoiler

There are only 83 total...and they are all under 100,000,000...

Spoiler

Also going with the "tree" approach that Molly Mae mentioned:

2
23
233
2333
23333
23339
2339
23399
233993
2339933
23399339
239
2393
2399
23993
239933
2399333
29
293
2939
29399
293999
2939999
29399999
3
31
311
3119
31193
313
3137
31379
317
37
373
3733
37337
373379
3733799
37337999
37339
373393
3739
37397
379
3793
3797
5
53
59
593
5939
59393
593933
5939333
59393339
59399
593993
599
7
71
719
7193
71933
719333
73
733
7331
7333
73331
739
7393
73939
739391
7393913
73939133
739393
7393931
7393933
739397
739399
79
797

Edited by Pickett
typos.

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