Thirty is the sum of the first four squares, which makes it a square pyramidal number. It is a primorial. 30 is the smallest sphenic number, and the smallest of the form, where r is a prime greater than 3. 30 has an aliquot sum of 42; the second sphenic number and all sphenic numbers of this form have an aliquot sum 12 greater than themselves. Adding up some subsets of its divisors gives 30, hence 30 is a semiperfect number. 30 is the largest number such that all coprimes smaller than itself, except for 1, are prime. A polygon with thirty sides is called a triacontagon. The icosahedron and the dodecahedron are Platonic solids with 30 edges. The icosidodecahedron is an Archimedean solid with 30 vertices, and the Tutte–Coxeter graph is a symmetric graph with 30 vertices. E8 has Coxeter number 30. 30 is a Harshad number. Since any group G such that |G| = pnm, where p does not divide m, has a subgroup of order pn, and 30 is the only number less than 60 that is not either a prime or of the above form, it is the only candidate for the order of a simple group less than 60 that one needs other methods to reject.
At about age 30, Jesus of Nazareth was baptized by John the Baptist, signaling the beginning of his public ministry of teaching and healing. It was also the age when David became King and Ezekiel and John the Baptist began their own ministries,
Age 30 is when Jewish priests traditionally start their service.
One of the rallying-cries of the 1960s student/youth protest movement was the slogan, 'Don't trust anyone over thirty'.
In The Myth of Sisyphus the French existentialist Albert Camus comments that the age of thirty is a crucial period in the life of a man, for at that age he gains a new awareness of the meaning of time.
In Franz Kafka's novel The Trial Joseph wakes up on the morning of his thirtieth birthday to find himself under arrest for an unspecified crime. After making many futile attempts to find the nature of the crime or the name of his accuser, Joseph dies on the eve of his thirty-first birthday.
The number of uprights that formed the Sarsen Circle at Stonehenge, also the supposed number of holes forming the arrays of Y and Z Holes at Stonehenge.
Western Christianity's most prolific 20th-century essayist, F. W. Boreham in 'Life at Thirty' mentions that in addition to Jesus commencing ministry at 30, Joseph was 30 when he stood before Pharaoh, King of Egypt, King David was 30 when he began to reign, and the Levites were numbered from the age of 30 and upward . Also in that essay Boreham writes 'It was said of Keats, that "he ensphered himself in thirty perfect years and died, not young".'
Sports
In tennis, the number 30 represents the second point gained in a game.
