Here is the definition of A279860 from OEIS:

a(n) is the first n-digit substring to repeat in the decimal expansion of e.

There is ongoing competition[1, 2] to find such repeated sequences in π, A197123, so I took a crack at the second-most-famous transcendental constant.

I searched the first 250 billion digits and found a(1)-a(22). a(23) was not found (I only had a 26.8% chance of finding it). I used a HashSet but would probably switch to a bloom filter for a(24) or higher. Rather than partitioning by prefix, I used the least significant bits of the sequence as a hash and used modulo partitioning.

Here are the sequences and their positions:

n a(n) First Position Repeat Position
1 2 0 4
2 18 2 6
3 182 2 6
4 1828 2 6
5 98793 478 494
6 987931 478 494
7 4349076 170 2731
8 82549802 11449 23544
9 450388721 9832 29424
10 5291493974 35210 159929
11 72883660263 55704 172254
12 476539957100 685615 947487
13 2557556217677 696773 4212711
14 45820090256930 937385 11527854
15 441340408673681 16824283 64070443
16 4413404086736815 16824283 64070443
17 04490816865108378 305179147 305519388
18 044908168651083787 305179147 305519388
19 3369635849169468729 3493534312 4849639160
20 29845348176416445308 15006890876 15441233099
21 561387588857094780748 33340537999 55184810049
22 8238609087161769234440 161691630698 176191766517