Arithmos is an AI-powered custom index builder. You type a sentence (“the 20 largest US semiconductor companies excluding any with debt-to-equity above 2” or “the AI core 3: NVIDIA, Microsoft, Amazon, equal-weighted”) and Arithmos returns a transparent, rules-based index with holdings, weights, rationale, a 10-year backtest against the S&P 500, and a one-click export to your broker. Below are nine concrete things people use it for that no off-the-shelf ETF lets you do.
What Arithmos actually is
Three layers, in plain English:
- An AI agent that translates a natural-language thesis into a structured index spec: a universe (which stocks are eligible), filters (market-cap thresholds, exclusions), and a weighting method (equal, market-cap, fundamentals).
- A backtest engine that prices every selected ticker daily for up to 25 years against your chosen benchmark (SPX, NDX, MSCI World, etc.) and surfaces CAGR, Sharpe, max drawdown, best year, worst year.
- A broker bridge that exports the holdings as a basket your existing brokerage account can execute in a single trade. No fund wrapper, no expense ratio, you own every share directly.
9 things you can build with Arithmos
1. Build a thematic index that doesn’t exist as an ETF
Want exposure to “the picks-and-shovels of sovereign AI buildouts: NVIDIA, TSMC, ASML, Vertiv, Eaton, Constellation Energy”? No ETF holds exactly that mix. With Arithmos you describe it once, hold the underlying shares, and Arithmos rebalances on whatever cadence you set. See our Sovereign AI Infrastructure Index for the worked example.
2. Explore rule-based vs active-management trade-offs
SPIVA scorecards consistently show that the majority of active managers underperform their benchmark over a 10-year window. Rules-based thematic baskets can exhibit lower turnover drag — the rule is the only thing trading. By way of historical example, our AI Core 3 Index (NVIDIA, Microsoft, Amazon, equal-weighted, quarterly rebalance) backtests at a 10Y CAGR of +53.5% versus SPX +12.6%, with a −31% max drawdown. That backtest benefits from hindsight — a 2014-era investor had no way to know AI infrastructure would dominate the decade. Forward results may differ materially. This is research, not a recommendation to buy any of these names.
3. Express ESG or values-based exclusions cleanly
Most ESG ETFs are blunt instruments. With Arithmos you can hold the S&P 500 excepttobacco, weapons, and any company with revenue exposure to thermal coal, and see exactly which 470 names you end up with rather than just “trust the issuer.”
4. Tax-loss harvesting at the individual stock level
Direct indexing makes single-name harvesting possible: when one of the 50 names in your basket dips, you can sell it at a loss and substitute a statistically similar name, banking the loss against future capital gains. Inside an ETF wrapper, that lever doesn’t exist. Tax treatment depends on your jurisdiction and personal circumstances — confirm with a qualified tax advisor before relying on this for tax planning.
5. Concentrate where you have conviction
ETF prospectuses cap single-name weights for diversification rules. If you genuinely believe NVIDIA, AMD, and TSMC dominate the next decade of compute, you can build a 3-stock equal-weight basket and hold exactly that. See Pure Semiconductor Monopoly.
6. Geo-target exposure that ETFs underrepresent
US-listed AI infrastructure, India manufacturing renaissance, Saudi Vision 2030 build-out, ASEAN consumer growth, Korea memory: you can build country-or-region indices targeted at the slices of the global market you care about, weighted however you want.
7. Replicate a hedge fund factor strategy without paying 2/20
Quality, low-vol, and momentum, screened for free cash flow positive every year for the last decade. These are the kinds of multi-factor rules quant hedge funds run. Describe them in English; Arithmos constructs the index.
8. Track an event or megatrend in real time
GLP-1 obesity drugs, drone warfare, satellite internet, stablecoin infrastructure: name the megatrend, get the basket. We update our official thematic indices as new entrants list.
9. Run yourself a backtest of an idea
Arithmos is a research and data platform. Use it to explore the historical behaviour of rules-based baskets — “how would an equal-weighted AI infrastructure basket have behaved over five years?” — without forming personal recommendations. Any investment decision remains entirely yours.
Arithmos vs ETFs vs robo-advisors
| Capability | ETFs | Robo-advisor | Arithmos |
|---|---|---|---|
| Custom thesis | No | No | Yes |
| Direct ownership of shares | No | Sometimes | Yes |
| Tax-loss harvest single names | No | Sometimes | Yes |
| Backtest before buying | No | No | Yes |
| Annual fee | 5–95 bps | 25 bps | Flat sub |
| Minimum | $0 | $500 | $0 |
How it works under the hood
Arithmos is a transparent stack. There is no black box.
- In-house tool-using agent. Your prompt routes to our portfolio-construction agent, which uses structured tools to screen stocks, fetch fundamentals, and finalise the spec. Each call is logged so we can show you exactly why each stock was picked.
- Real market data.Prices, fundamentals, and benchmarks come from Financial Modeling Prep with a multi-provider fallback chain (Yahoo, Tiingo). No hand-curated “model portfolios.”
- Open methodology. Every official Arithmos index ships with the prompt that built it, the rebalance cadence, and a public-readable holdings page.
Who it’s for
- Self-directed investors who’ve outgrown SPY/QQQ.
- Wealth managers who want a thematic builder for client conversations.
- Quant-curious retail looking to express a thesis without coding.
- Anyone running a tax-aware brokerage account.
FAQ
Does Arithmos give financial advice?
No. Arithmos is a research and construction tool. Past backtest performance does not guarantee future results.
Where do the prices come from?
Financial Modeling Prep (primary), with Yahoo Finance and Tiingo as automatic fallbacks if the primary doesn’t cover a ticker.
Can I export to my broker?
Yes. Arithmos generates broker-ready basket files (CSV, IBKR, M1, Fidelity formats) on Plus and Pro plans.
How is this different from a robo-advisor?
Robo-advisors are regulated services that pick a strategy and place trades on your behalf. Arithmos is the opposite: an unregulated research and data tool. You pick the brief, the engine does the construction and backtest, and any trading is something you do yourself through your own broker. Nothing here is investment advice.
