# 70 (number)

**70** (seventy) is a natural number whose prime factorization is 2 × 5 × 7, which makes it the fourth sphenic number, after 30, 42, and 66.<sup>[1](https://proofwiki.org/wiki/70)</sup> It is best known in mathematics as the smallest weird number, the first of the sequence 70, 836, 4030, 5830, ...<sup>[2](https://oeis.org/A006037)</sup> In science, 70 is the atomic number of ytterbium, a lanthanide.<sup>[3](http://encyclopedia.tfd.com/70+(number))</sup>

| Key fact | Detail |
|---|---|
| Factorization | 70 = 2 × 5 × 7; fourth sphenic number after 30, 42, 66<sup>[1](https://proofwiki.org/wiki/70)</sup> |
| Divisors | 1, 2, 5, 7, 10, 14, 35, 70; divisor sum σ = 144 = 12²; totient φ = 24<sup>[4](https://oeis.org/A018270/internal)</sup><sup> • </sup><sup>[1](https://proofwiki.org/wiki/70)</sup> |
| Weird number | Smallest weird number: abundant (aliquot sum 74) but no subset of proper divisors sums to 70; next is 836<sup>[5](https://mathworld.wolfram.com/WeirdNumber.html)</sup><sup> • </sup><sup>[2](https://oeis.org/A006037)</sup> |
| Combinatorics | 70 = C(8,4), the fourth central binomial coefficient and fifth pentatope number<sup>[1](https://proofwiki.org/wiki/70)</sup> |
| Other classifications | Happy number, Harshad number (digit sum 7), Pell number, seventh pentagonal number, Erdős–Woods number<sup>[1](https://proofwiki.org/wiki/70)</sup><sup> • </sup><sup>[3](http://encyclopedia.tfd.com/70+(number))</sup> |
| Element | Atomic number 70 is ytterbium (Yb), a period-6 lanthanide<sup>[6](https://number.wiki/70)</sup> |
| Cultural uses | Seventy elders, seventy nations of Genesis 10, seventy disciples, seventy years of Babylonian exile, "three score and ten" years of life<sup>[3](http://encyclopedia.tfd.com/70+(number))</sup> |

## In mathematics

The divisors of 70 are 1, 2, 5, 7, 10, 14, 35, and 70. They add up to 144, more than twice 70, so 70 is abundant.<sup>[4](https://oeis.org/A018270/internal)</sup> The divisor sum is a perfect square, σ(70) = 144 = 12², and 70 is the fifth number with this property after 1, 3, 22, and 66; it has eight divisors (σ₀ = 8) and a totient φ(70) = 24.<sup>[1](https://proofwiki.org/wiki/70)</sup> Its aliquot sequence runs 70 → 74 → 40 → 50 → 43 → 1 → 0, terminating at zero.<sup>[6](https://number.wiki/70)</sup>

Beyond abundance, 70 carries a long list of classifications: it is a happy number, a [Harshad number](https://www.edgepedia.org/harshad-number) (it is a multiple of its digit sum 7), a Pell number, the seventh pentagonal number, the fourth tridecagonal number, and an Erdős–Woods number, meaning sequences of 70 consecutive integers exist in which each inner member shares a factor with the first or the last member.<sup>[1](https://proofwiki.org/wiki/70)</sup><sup> • </sup><sup>[3](http://encyclopedia.tfd.com/70+(number))</sup> It is also the second primitive abundant number after 20.<sup>[1](https://proofwiki.org/wiki/70)</sup>

## Weird number status

A weird number is abundant without being pseudoperfect (semiperfect): the sum of its proper divisors exceeds the number, yet no subset of those divisors sums to the number itself.<sup>[5](https://mathworld.wolfram.com/WeirdNumber.html)</sup> For 70 the proper divisors 1, 2, 5, 7, 10, 14, and 35 sum to 74, four more than 70, but no subset of them totals 70.<sup>[5](https://mathworld.wolfram.com/WeirdNumber.html)</sup><sup> • </sup><sup>[7](https://proofwiki.org/wiki/Weird_Number/Examples/70)</sup>

The obstruction is the missing 4. Since 70 = 74 − 4, a subset summing to 70 would require the remaining divisors to sum to 4, and 4 cannot be made from the smaller divisors: only 1 and 2 lie below 4, and 1 + 2 = 3.<sup>[4](https://oeis.org/A018270/internal)</sup><sup> • </sup><sup>[8](https://www.matpic.com/essays/abundant-and-still-not-a-sum-of-its-parts/)</sup> Equivalently, in the product ∏(1 + x^d) over 70's proper divisors, every exponent from 0 to 74 appears except 4 and 70; a number is semiperfect exactly when its own exponent appears.<sup>[8](https://www.matpic.com/essays/abundant-and-still-not-a-sum-of-its-parts/)</sup> This contrasts with the smallest abundant number, 12, whose proper divisors 1, 2, 3, 4, 6 include the subset 6 + 4 + 2 = 12, so 12 is abundant but not weird.<sup>[5](https://mathworld.wolfram.com/WeirdNumber.html)</sup>

Stanley Benkoski named such numbers weird in 1972, and Benkoski and [Paul Erdős](https://www.edgepedia.org/paul-erdos) proved in 1974, in *Mathematics of Computation*, that weird numbers have positive density: a definite small proportion of all integers is weird.<sup>[8](https://www.matpic.com/essays/abundant-and-still-not-a-sum-of-its-parts/)</sup><sup> • </sup><sup>[9](https://www.ams.org/journals/mcom/1974-28-126/S0025-5718-1974-0347726-9/)</sup> Their paper also left two questions open: whether any odd weird numbers exist, and whether σ(n)/n can be arbitrarily large for weird n.<sup>[9](https://www.ams.org/journals/mcom/1974-28-126/S0025-5718-1974-0347726-9/)</sup> All known weird numbers are even; one source records 10^17 as an accepted lower bound for an odd weird number, while MathWorld credits W. Fang (2013) with a lower bound it does not state numerically.<sup>[10](https://planetmath.org/weirdnumber)</sup><sup> • </sup><sup>[5](https://mathworld.wolfram.com/WeirdNumber.html)</sup>

Weird numbers are sparse in absolute terms: only 7 exist up to 10^4, and 36 lie below 20,000, a proportion of about one in 550.<sup>[11](https://ar5iv.labs.arxiv.org/html/1802.07178)</sup><sup> • </sup><sup>[8](https://www.matpic.com/essays/abundant-and-still-not-a-sum-of-its-parts/)</sup> They are nevertheless infinite: if n is weird and p is a prime larger than σ(n), then np is weird.<sup>[11](https://ar5iv.labs.arxiv.org/html/1802.07178)</sup> Since σ(70) = 144, every number 70p with prime p > 144 is weird; examples include 18830 = 2·5·7·269, 18970 = 2·5·7·271, and 19390 = 2·5·7·277.<sup>[12](https://mathoverflow.net/questions/430063/weirdness-in-the-sequence-the-number-of-divisors-for-a-weird-number)</sup> The next weird number after 70 is 836, followed by 4030, 5830, 7192, 7912, and 9272.<sup>[2](https://oeis.org/A006037)</sup>

## Combinatorial appearances

Seventy counts the ways to choose 4 objects out of 8: it is the binomial coefficient C(8,4) = 8!/(4!)², the fourth central binomial coefficient after 2, 6, and 20.<sup>[1](https://proofwiki.org/wiki/70)</sup><sup> • </sup><sup>[13](https://www.numbersaplenty.com/70)</sup> It is also the fifth pentatope number, after 1, 5, 15, and 35.<sup>[1](https://proofwiki.org/wiki/70)</sup><sup> • </sup><sup>[3](http://encyclopedia.tfd.com/70+(number))</sup>

## How it compares with neighboring integers

Seventy sits between the primes 67 and 71; its other immediate neighbor, 69, is composite.<sup>[13](https://www.numbersaplenty.com/70)</sup> Among sphenic numbers it is the fourth, in the run 30, 42, 66, 70, 78, 102, 105, 110, 114; a sphenic number is a product of exactly three distinct primes, and every such number has precisely eight positive divisors.<sup>[14](https://mathworld.wolfram.com/SphenicNumber.html)</sup>

## Base systems and digit properties

It is nontrivially palindromic in three bases: 77 in base 9, 55 in base 13, and 22 in base 34.<sup>[3](http://encyclopedia.tfd.com/70+(number))</sup><sup> • </sup><sup>[13](https://www.numbersaplenty.com/70)</sup> As a Harshad number in base 10 it is divisible by its digit sum 7, and it is a Harshad number in bases 6, 8, 9, 10, 11, 13, 14, 15, and 16 as well.<sup>[3](http://encyclopedia.tfd.com/70+(number))</sup> One curiosity of digit sums: 70 is the largest known n such that 2^n has digit sum n, the only other known case being 5.<sup>[15](https://numbergossip.com/70)</sup> The same source also mentions a square-pyramid cannonball result.<sup>[15](https://numbergossip.com/70)</sup>

## In science

[Atomic number](https://www.edgepedia.org/atomic-number) 70 is ytterbium (Yb), a lanthanide in period 6 of the periodic table.<sup>[6](https://number.wiki/70)</sup><sup> • </sup><sup>[3](http://encyclopedia.tfd.com/70+(number))</sup>

## In culture and history

The number seventy recurs throughout the [Hebrew Bible](https://www.edgepedia.org/hebrew-bible) and early Jewish and Christian tradition. Moses assembled seventy elders (Numbers 11:16–30), seventy members of Jacob's family went down to Egypt (Genesis 46:27), and the Great Sanhedrin, the supreme Jewish council, was traditionally described as having seventy judges plus its presiding head.<sup>[3](http://encyclopedia.tfd.com/70+(number))</sup> In Jewish tradition the Torah lists seventy descendants of Noah after the Flood, from which the sages learn that humanity comprises seventy nations, each with its own language.<sup>[16](https://www.chabad.org/library/article_cdo/aid/940857/jewish/Why-Is-70-Special.htm)</sup> Hebrew manuscripts of Genesis 10 list seventy nations, while the Septuagint's Table of Nations lists seventy-two.<sup>[17](https://archive.bookofmormoncentral.org/sites/default/files/archive-files/pdf/brown/2021-07-08/04_s._kent_brown_the_seventy_in_scripture_25-44.pdf)</sup>

In the [New Testament](https://www.edgepedia.org/new-testament), Luke 10:1–24 records Jesus appointing seventy disciples; the number was symbolic to Jews because of the seventy elders and the Sanhedrin's membership.<sup>[18](https://www.biblegateway.com/resources/encyclopedia-of-the-bible/Seventy-Disciples)</sup> Manuscripts of Luke 10 vary between seventy and seventy-two, and the twelve disciples were associated with the tribes of Israel while the seventy were associated with the gentile nations of the earth.<sup>[17](https://archive.bookofmormoncentral.org/sites/default/files/archive-files/pdf/brown/2021-07-08/04_s._kent_brown_the_seventy_in_scripture_25-44.pdf)</sup> Academic scholarship connects the seventy elders of Moses to the later legend of the seventy (or seventy-two) translators of the [Septuagint](https://www.edgepedia.org/septuagint) as told in the Letter of Aristeas, Philo, and Josephus.<sup>[19](https://etheses.dur.ac.uk/11078/1/Moses_and_the_Seventy_Elders_(corrected).pdf?DDD32+)</sup>

The Hebrew word שִׁבְעִים (shib'im, Strong's 7657), built on the root for seven, carries the seventy-year lifespan of Psalm 90:10, Jeremiah's seventy-year Babylonian exile (Jeremiah 25:11; 29:10), and Daniel's seventy weeks (Daniel 9:24–27).<sup>[20](https://www.biblehub.com/hebrew/7657.htm)</sup> Psalm 90:10's allotment survives in English as "three score and ten" years for a human life.<sup>[3](http://encyclopedia.tfd.com/70+(number))</sup>

## Everyday measurement

Seventy miles per hour is the national speed limit in the United Kingdom for cars and motorcycles on the best grades of road, and a common freeway limit in the United States; seventy years of marriage is the platinum wedding anniversary.<sup>[3](http://encyclopedia.tfd.com/70+(number))</sup>

## Recent research (2024–2025)

Weird numbers remain active in current number theory, with 70 serving as the canonical example. A 2024 paper in *INTEGERS* by Joshua Zelinsky, a mathematician writing on near-perfect numbers, restates the case: σ(70) = 144 > 140, yet 70 is not s-near perfect for any s, so it is weird, and whether odd weird numbers exist is a classic open problem.<sup>[21](https://math.colgate.edu/~integers/y61/y61.pdf)</sup> A 2025 arXiv preprint makes the same point, noting that abundance does not imply k-near perfection for any k, with 70 as the counterexample, and crediting the class's investigation to Benkoski and Erdős.<sup>[22](https://arxiv.org/pdf/2508.06651)</sup> These papers concern the weird-number class generally; no new results specific to 70 itself appear in them.

## References

1. [70 — ProofWiki](https://proofwiki.org/wiki/70)
2. [A006037 — Weird numbers, OEIS](https://oeis.org/A006037)
3. [70 (number) — TheFreeDictionary encyclopedia](http://encyclopedia.tfd.com/70+(number))
4. [A018270 — Divisors of 70, OEIS](https://oeis.org/A018270/internal)
5. [Weird Number — Wolfram MathWorld](https://mathworld.wolfram.com/WeirdNumber.html)
6. [70 — events, facts & calendars — NumberWiki](https://number.wiki/70)
7. [Weird Number/Examples/70 — ProofWiki](https://proofwiki.org/wiki/Weird_Number/Examples/70)
8. [Abundant, and still not a sum of its parts — Matpic](https://www.matpic.com/essays/abundant-and-still-not-a-sum-of-its-parts/)
9. [Benkoski & Erdős (1974). On Weird and Pseudoperfect Numbers. Mathematics of Computation 28.](https://www.ams.org/journals/mcom/1974-28-126/S0025-5718-1974-0347726-9/)
10. [Weird number — PlanetMath](https://planetmath.org/weirdnumber)
11. [Primitive abundant and weird numbers with many prime factors — arXiv](https://ar5iv.labs.arxiv.org/html/1802.07178)
12. [Weirdness in the sequence of divisor counts of weird numbers — MathOverflow](https://mathoverflow.net/questions/430063/weirdness-in-the-sequence-the-number-of-divisors-for-a-weird-number)
13. [Properties of number 70 — Numbers Aplenty](https://www.numbersaplenty.com/70)
14. [Sphenic Number — Wolfram MathWorld](https://mathworld.wolfram.com/SphenicNumber.html)
15. [70 — Number Gossip](https://numbergossip.com/70)
16. [Why Is 70 Special? — Chabad.org](https://www.chabad.org/library/article_cdo/aid/940857/jewish/Why-Is-70-Special.htm)
17. [S. Kent Brown, The Seventy in Scripture](https://archive.bookofmormoncentral.org/sites/default/files/archive-files/pdf/brown/2021-07-08/04_s._kent_brown_the_seventy_in_scripture_25-44.pdf)
18. [Seventy Disciples — Encyclopedia of The Bible, Bible Gateway](https://www.biblegateway.com/resources/encyclopedia-of-the-bible/Seventy-Disciples)
19. [Moses and the Seventy Elders — Durham University thesis](https://etheses.dur.ac.uk/11078/1/Moses_and_the_Seventy_Elders_(corrected).pdf?DDD32+)
20. [Strong's Hebrew 7657: שִׁבְעִים (shib'im) — Seventy — Bible Hub](https://www.biblehub.com/hebrew/7657.htm)
21. [Zelinsky, On 2-Near Perfect Numbers, INTEGERS 24 (2024)](https://math.colgate.edu/~integers/y61/y61.pdf)
22. [Weird and k-near perfect numbers — arXiv preprint (2025)](https://arxiv.org/pdf/2508.06651)

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*Topic: Encyclopedia › Physical world and mathematics › Mathematics and statistics › Numbers and algebra › Arithmetic and number systems › Integer sequences and partitions › Special and named integers › Named individual integers*

*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*

*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*

License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license