Source. Appears to be a compilation of several videos, presumably each with their own separate LA-UR, though these are not noted. Credited: "Video courtesy of the Los Alamos National Laboratory Archives", TODO how to search that archive online?
Frame: size: XS. Marking near bottom bracket: F/N GY17Z1327 DK: WGI GY17Z1327 F ES:BI-2339.
Seatpost: TODOdiameter. Measured in store 30.4mm after original stolen, although 2020 says 30.9. 30.4mm seemed to fit OK, and measuring with ruler gives 30.5/30.6, I don't think a 30.9 can possibly fit.
Sometimes you get annoyed to death with your bike not breaking or changing gears perfectly as you would like, and the people at the bike shop never do the job well enough.
The problem with bike shops is that the employees are already swamped with work, and they don't get paid any extra for doing more work.
Asa result, paradoxically, they are often happier, and respect you more if you are trying to get them to help you to fix your own bike!
Also, for the same reason, they don't have the time to go for a quick test ride after a fix to ensure that the bug was actually fixed.
So they ignore things that would obviously be huge ridability benefits (although they might not be obvious to newbie customers), for which customers would gladly pay more money for.
But you start to learn how to do stuff yourself and it feel amazing when you finally get there (after infinite trial and error).
Ciro dreams of a bike shop that actually calls you for the appointment and then teaches you how to fix the thing.
So the best strategy is to have a backup bicycle, and when your main one breaks, you just try to to the fix yourself. That means:
Then, if you fail to do the fix, that is OK, just take it to the bike shop, with the piece you've bought, and ask them to do it for you. At least this way you did not wastea golden opportunity to learn!
The model is extremely simple, but has been proven to be able to solve all the problems that any reasonable computermodel can solve, thus its adoption as the "default model".
Also interestingly, we see that both of those reaction require some extra energy to catalyse, one needing adenosine triphosphate and the other nADP+.
TODO: any mention of how much faster it makes the reaction, numerically?
Since this is an enzyme, it would also be interesting to have a quick search for it in the KEGG entry starting from the organism: www.genome.jp/pathway/eco01100+M00022We type in the search bar "thrA", it gives a long list, but the last entry is our "thrA". Selecting it highlights two pathways in the large graph, so we understand that it catalyzes two different reactions, as suggested by the proteinname itself (fused blah blah). We can now hover over:
the edge: it shows all the enzymes that catalyze the given reaction. Both edges actually have multiple enzymes, e.g. the L-Homoserine path is also catalyzed by another enzyme called metL.
the node: they are the metabolites, e.g. one of the paths contains "L-homoserine" on one node and "L-aspartate 4-semialdehyde"
Note that common cofactor are omitted, since we've learnt from the UniProt entry that this reaction uses ATP.
Aspartate kinase I / homoserine dehydrogenase I comprises a dimer of ThrA dimers. Although the dimeric form is catalytically active, the binding equilibrium dramatically favors the tetrameric form. The aspartate kinase and homoserine dehydrogenase activities of each ThrA monomer are catalyzed by independent domains connected by a linker region.
0inf denotes the right or left edge of the (zero initialized) tape. It is often omitted aswe always just assume it is always present on both sides of every regex
A, B, C, D and E denotes the current machine state. This is especially common notation in the context of the BB(5) problem
< and > next to the state indicate if the head is on top of the left or right element. E.g.:
11 (01)^n <A 00 (0011)^{n+2}
indicates that the head A is on top of the last 1 of the last sequence of n 01s to the left of the head.
each transaction (transaction is often abbreviated "tx") has a list of inputs, and a list of outputs
each input is the output of a previous transaction. You verify your identity as the indented receiver by producing a digital signature for the public key specified on the output
each output specifies the public key of the receiver and the value being sent
the sum of output values cannot obvious exceed the sum of input values. If it is any less, the leftover is sent to the miner of the transaction asa transaction fee, which is an incentive for mining.
once an output is used from an input, it becomes marked as spent, and cannot be reused again. Every input uses the selected output fully. Therefore, if you want to use an input of 1 BTC to pay 0.1 BTC, what you do is to send 0.1 BTC to the receiver, and 0.9 BTC back to yourself as change. This is why the vast majority of transactions has two outputs: one "real", and the other change back to self.
this means that there was 0.2 left over from the input. This value will be given to the miner that mines this transaction.
Since this is a regular transaction, no new coins are produced.
tx2: regular transaction with a single input and a single output. It uses up the entire input, leading to 0 miner fees, so this greedy one might (will?) never get mined.
tx3: regular transaction with two inputs and one output. The total input is 2.3, and the output is 1.8, so the miner fee will be 0.5
Since every input must come from a previous output, there must be some magic way of generating new coins from scratch to bootstrap the system. This mechanism is that when the miner mines successfully, they get amining fee, which is a magic transaction without any valid inputs and a pre-agreed value, and an incentive to use their power/compute resources to mine. This magic transaction is called a "coinbase transaction".
The key innovation of Bitcoin is how to prevent double spending, i.e. use a single output as the input of two different transactions, via mining.
For example, what prevents me from very quickly using a single output to pay two different people in quick succession?
The solution are the blocks. Blocks discretize transactions into chunks in a way that prevents double spending.
a list of transactions that are valid amongst themselves. Notably, there can't be double spending within a block.
People making transactions send them to the network, and miners select which ones they want to add to their block. Miners prefer to pick transactions that are:
small, as less bytesmeans less hashing costs. Small generally means "doesn't have a gazillion inputs/outputs".
the ID of its parent block. Blocks therefore form a linear linked list of blocks, except for temporary ties that are soon resolved. The longest known list block is considered to be the valid one.
a nonce, which is an integer chosen "arbitrarily by the miner"
For a block to be valid, besides not containing easy to check stuff like double spending, the miner must also select a nonce such that the hash of the block starts with N zeroes.
The nonces are on this example arbitrary chosen numbers that would lead to a desired hash for the block.
block0 is the Genesis block, which is magic and does not have a previous block, because we have to start from somewhere. The network is hardcoded to accept that asa valid starting point.
Now suppose that the person who created tx2 had tried to double spend and also created another transaction tx2' at the same time that looks like this:
Clearly, this transaction would try to spend tx0 out1 one more time in addition to tx2, and should not be allowed! If this were attempted, only the following outcomes are possible:
block1 contains tx2. Then when block2 gets made, it cannot contain tx2', because tx0 out1 was already spent by tx2
block1 contains tx2'. tx2 cannot be spent anymore
Notably, it is not possible that block1 contains both tx2 and tx2', as that would make the block invalid, and the network would not accept that block even if a miner found a nonce.
Since hashes are basically random, miners just have to try abunch of nonces randomly until they find one that works.
The more zeroes, the harder it is to find the hash. For example, on the extreme case where N is all the bits of the hash output, we are trying to find a hash of exactly 0, which is statistically impossible. But if e.g. N=1, you will in average have to try only two nonces, N=2 four nonces, and so on.
The value N is updated every 2weeks, and aims to make blocks to take 10minutes to mine on average. N has to be increased with time, as more advanced hashing hardware has become available.
Once a miner finds a nonce that works, they send their block to the network. Other miners then verify the block, and once they do, they are highly incentivized to stop their hashing attempts, and make the new valid block be the new parent, and start over. This is because the length of the chain has already increased: they would need to mine two blocks instead of one if they didn't update to the newest block!
Therefore if you try to double spend, some random miner is going to select only one of your transactions and add it to the block.
They can't pick both, otherwise their block would be invalid, and other miners wouldn't accept is as the new longest one.
Then sooner or later, the transaction will be mined and added to the longest chain. At this point, the network will move to that newer header, and your second transaction will not be valid for any miner at all anymore, since it usesa spent output from the first one that went in. All miners will therefore drop that transaction, and it will never go in.
The goal of having this mandatory 10minutes block interval is to make it very unlikely that two miners will mine at the exact same time, and therefore possibly each one mine one of the two double spending transactions. When ties to happen, miners randomly choose one of the valid blocks and work on top of it. The first one that does, now has a block of lengthL + 2 rather than L + 1, and therefore when that is propagated, everyone drops what they are doing and move to that new longest one.
