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In recreational number theory, a minimal prime is a prime number for which there is no shorter subsequence of its digits in a given base that form a prime. In base 10 there are exactly 15 minimal primes:

2, 3, 5, 7, E, 11, 61, 81, 91, 401, X41, 4441, X0X1, XXXX1, 44XXX1, XXX0001, XX000001

Similarly, there are exactly 3X composite numbers which have no shorter composite subsequence:

4, 6, 8, 9, X, 10, 12, 13, 20, 21, 22, 23, 2E, 30, 32, 33, 50, 52, 53, 55, 70, 71, 72, 73, 77, 7E, E0, E1, E2, E3, EE, 115, 151, 15E, 257, 275, 311, 317, 31E, 351, E57, E75, 1111, 1117, 111E, 5111

All minimal primes are found for all bases up to 20 except 15, 17 and 19. All these three bases have only one unsolved family:

Base The only one unsolved family Form (“n” is the number of digits in “{}”)
15 13:1:{9} (2461*15^n-9)/14
17 12:12:1:{6} (92X4*17^n-1)/3
19 14:{0}:13:18 4100*19^n+23E

Besides, there are large probable prime found for these bases, e.g. in base 1E, the largest minimal (probable) prime is 9:{12}327575 (=(8X*1E^327575-7)/E), and in base 19, a large minimal (probable) prime is 10:{13}1E134E:0:18 (=(43*19^1E134E-877)/4), this prime is likely the second-largest “base 19 minimal prime”.

However, this problem seems to be hard for bases > 20, e.g. in base 21, there are 10 unsolved families, and although this problem is solved for all bases <= 20 except 15, 17 and 19, but this problem is unsolved for all bases > 20 and <= 40 except 26 and 36. (this problem has only reserved for bases <= 40)

The bases > 20 and <= 40 with only few unsolved families are

Base Unsolved families
22 {X}:6:13

{16}:14:19

24 20:{X}:13
26 (none)
30 20:{19}:2E

{21}:24:2E

34 22:30:{26}:29

24:{22}:33

36 (none)

Even in base 11, the largest minimal (probable) prime is also large, it is 8:{0}16641:1:1:1, which is equal to X208*11^16641+133.

In bases 11, 15, 17, 19 and 1E (also all bases > 20 and <= 40 except 26 and 36), some minimal primes found are only probable primes, i.e. not proved primes. (For all bases <= 20 except 11, 15, 17, 19 and 1E, also for bases 26 and 36, all minimal primes found are proved primes)

For bases up to 10, the largest minimal primes are:

Base Largest minimal prime Length
1 11 2
2 11 2
3 111 3
4 11 2
5 44441 5
6 40041 5
7 11111 5
8 444444441 9
9 1101 4
X 66600049 8
E 444444444444444444444444444444444444444444441 39
10 XX000001 8

The known largest minimal (probable) primes for some bases b > 10 are:

Base Largest minimal (probable) prime Length Dozenal length
11 8:{0}16641:1:1:1 16645 17165
12 4:{0}6E:4:9 72 77
13 9:{6}88:0:8 8E 99
14 9:{0}2072:9:1 2075 2358
16 14:14:{0}30:1 29 33
18 {14}313:9:9 315 392
1X 18:{0}534:12:10:1 538 673
1E 9:{12}327575 327576 408990
20 1X:{6}82:1 84 X8
26 10:{0}712:1 714 98X
36 {23}346:1 347 511

For these (probable) primes, only the largest minimal (probable) prime in bases 11, 14 and 1E are titanic primes (primes with at least 1000 dozenal digits), that in bases 11 and 1E are also gigantic primes (primes with at least 10000 dozenal digits), however, none of them are megaprimes (primes with at least 106 dozenal digits).

The minimal primes in bases up to 10 are

b minimal primes in base b (written in base b) number of minimal primes in base b
1 11 1
2 10, 11 2
3 2, 10, 111 3
4 2, 3, 11 3
5 2, 3, 10, 111, 401, 414, 14444, 44441 8
6 2, 3, 5, 11, 4401, 4441, 40041 7
7 2, 3, 5, 10, 14, 16, 41, 61, 11111 9
8 2, 3, 5, 7, 111, 141, 161, 401, 661, 4611, 6101, 6441, 60411, 444641, 444444441 13
9 2, 3, 5, 7, 14, 18, 41, 81, 601, 661, 1011, 1101 10
X 2, 3, 5, 7, 11, 19, 41, 61, 89, 409, 449, 499, 881, 991, 6469, 6949, 9001, 9049, 9649, 9949, 60649, 666649, 946669, 60000049, 66000049, 66600049 22
E 2, 3, 5, 7, 10, 16, 18, 49, 61, 81, 89, 94, 98, 9X, 199, 1XX, 414, 919, X1X, XX1, 11X9, 66X9, X119, X911, XXX9, 11144, 11191, 1141X, 114X1, 1411X, 144X4, 14X11, 1X114, 1X411, 4041X, 40441, 404X1, 4111X, 411X1, 44401, 444X1, 44X01, 6X609, 6X669, 6X696, 6X906, 6X966, 90901, 99111, X0111, X0669, X0966, X0999, X0X09, X4401, X6096, X6966, X6999, X9091, X9699, X9969, 401X11, 404001, 404111, 440X41, 4X0401, 4X4041, 60X069, 6X0096, 6X0X96, 6X9099, 6X9909, 909991, 999901, X00009, X60609, X66069, X66906, X69006, X90099, X90996, X96006, X96666, 111114X, 1111X14, 1111X41, 1144441, 14X4444, 1X44444, 4000111, 4011111, 41X1111, 4411111, 444441X, 4X11111, 4X40001, 6000X69, 6000X96, 6X00069, 9900991, 9990091, X000696, X000991, X006906, X040041, X141111, X600X69, X906606, X909009, X990009, 40X00041, 60X99999, 99000001, X0004041, X9909006, X9990006, X9990606, X9999966, 40000X401, 44X444441, 900000091, X00990001, X44444111, X66666669, X90000606, X99999006, X99999099, 600000X999, X000144444, X900000066, X0000000001, X0014444444, 40000000X0041, X000000014444, X044444444441, X144444444411, 40000000000401, X0000044444441, X00000000444441, 11111111111111111, 14444444444441111, 44444444444444111, X1444444444444444, X9999999999999996, 1444444444444444444, 4000000000000000X041, X999999999999999999999, X44444444444444444444444441, 40000000000000000000000000041, 440000000000000000000000000001, 999999999999999999999999999999991, 444444444444444444444444444444444444444444441 108
10 2, 3, 5, 7, E, 11, 61, 81, 91, 401, X41, 4441, X0X1, XXXX1, 44XXX1, XXX0001, XX000001 15