A 18.93M parameter decoder-only transformer trained from scratch on the DeepMind Mathematics Dataset, character level, in 45 minutes on a single RTX 4060 laptop GPU.

This is a learning-by-doing project: no transformers, no lightning, no config framework. Tokenizer, data pipeline, model, training loop and evaluation are all hand-written. Source: https://github.com/Rekhii/ChatGPT

Answers maths questions phrased exactly the way the DeepMind dataset phrases them:

What is the first derivative of 13*a**2 - 627434*a + 1191410?  ->  26*a - 627434
Sort 53, -2.2, -3, 14, 36 in increasing order.                 ->  -3, -2.2, 14, 36, 53
What is the hundred thousands digit of 82923295?               ->  9

It is not a chat model. It saw one format during training, question<sep>answer<eos>, and nothing else. Free-form phrasing produces garbage.

Exact string match on 200 held-out questions per module, final checkpoint (step 7385).

Final validation loss 0.339, down from 4.705 at initialisation.

The model learned every task expressible as a fixed-depth transformation and failed every task requiring iterative search.

comparison__sort (85.5%) and numbers__gcd (33.5%) are both integer tasks, but sorting is comparison and gcd requires factorization. numbers__is_prime sat at 50.0%, 50.0%, 50.5%, 54.0%, 49.0%, 56.0%, 49.0%, 50.0% across all eight checkpoints, perfectly flat for 7,386 steps while every other module climbed. Primality cannot be pattern-matched from digit strings, and a fixed-depth forward pass with no scratchpad has nowhere to run a factorization loop.

Failure modes are also informative. On gcd(1569427, 657) the model answers 657, having learned the shortcut "when one number is much smaller, the gcd is often that number", which is true whenever the smaller divides the larger. On gcd(464690, 48980) it answers 2210 against a true 310: wrong, but the right order of magnitude. It estimates the size of a plausible answer without being able to compute it.

Arithmetic errors show the classic character-level signature. -125266 - 1686191 returns -1707797 against a true -1811457: correct sign, correct digit count, wrong middle digits. Meanwhile -2769526541 - -0.5 is exactly right. Alignment and formatting are reliable; carry propagation is not.

Character level was chosen over BPE deliberately: digit-level tokenization gives the model positional resolution over numbers that a subword vocabulary destroys.

Only 14.9% of tokens are answer tokens. Loss is masked so the model is not rewarded for autocompleting question phrasing.

max_steps was derived from measurement rather than guessed: a 15 minute sustained throughput benchmark run to thermal equilibrium (86-87C, SM clock stable at 1920-2100 MHz) gave 134,460 tok/s, and the step count was set from that. Predicted 45 minutes, actual 45.2.

The checkpoint is a raw state_dict, not a transformers model. You need model.py, config.py and tokenizer.py from the GitHub repo.

Generate with batch size 1, or group prompts by identical length. The model never saw <pad> during training, so left-padded batches put it in a state it has no representation for and generation degrades badly. In this project that bug alone cost 6.7x on the headline number: the same checkpoint scored 6.5% padded and 43.5% unpadded.

Decode greedily. There is one correct answer to an arithmetic question, so sampling is strictly worse.

Roughly 17 passes over 21M tokens rather than one pass over 363M. Chinchilla-optimal for this parameter count is around 380M tokens, so the run was compute-matched but data-repeated. Some of the reported accuracy may be memorisation, which the held-out eval set bounds but does not eliminate.

Trained on easy and medium difficulty splits only. Hard was excluded.

arithmetic__add_or_sub, arithmetic__div and arithmetic__mul were still gaining roughly 5 points per 1,000 steps when the clock ran out. Those numbers are a floor.

Not a calculator, not a chat model, and no use for anything beyond understanding how small transformers learn structured tasks.