15

I've noticed this number at least in two places:

The number looks suspiciously familiar, and I keep asking myself:
"Where have I seen it before?" and "What exactly is this number?"

  • 1
    It's "pi million." Pi = 3.141592 – Tjaden Hess Jan 24 '16 at 21:10
  • @TjadenHess Oh! That explains it all... – Kostya Jan 24 '16 at 21:15
  • 2
    For the record, it's an incorrectly rounded approximation to pi. The approximation would be slightly more accurate if the final digit was changed to a 3 instead of just directly copied from the longer decimal expansion. /pedant – Jeff Coleman Jan 25 '16 at 3:04
  • 1
    In Homestead, the gasLimit is now 4,712,388 (1.5 times PI million). – eth May 31 '16 at 9:58
17

This is referred to as a "Nothing up my sleeve number" which is common practice for security / cryptography based software.

From Wikipedia:

In cryptography, nothing up my sleeve numbers are any numbers which, by their construction, are above suspicion of hidden properties. They are used in creating cryptographic functions such as hashes and ciphers. These algorithms often need randomized constants for mixing or initialization purposes. The cryptographer may wish to pick these values in a way that demonstrates the constants were not selected for a nefarious purpose, for example, to create a backdoor to the algorithm. These fears can be allayed by using numbers created in a way that leaves little room for adjustment. An example would be the use of initial digits from the number π as the constants. Using digits of π millions of places into its definition would not be considered as trustworthy because the algorithm designer might have selected that starting point because it created a secret weakness the designer could later exploit.

  • 2
    Interesting, knew it was pi million but thought it was just for the nerdy factor. Bolded the PI part in your answer. – eth Jan 24 '16 at 22:30
5

In addition to the answer explaining nothing up my sleeve number of pi million, it's worth to add that the gas limit is not set in stone.

As the yellow paper equations describe the gas limit can adjust with every block plusminus 1/1024 which is around 0.09%.

In case of a very high network utilization, the network could adjust the gas limit by a factor of around 131 per day, assuming there are around 5k blocks per day:

1.0009765625 ^ 5000 = 131.68725950103537531831

See also live, what happens if you increase the gas limit:

gas limit increase

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