18145

Appears in sequences

• Doubly triangular numbers: a(n) = n*(n+1)*(n^2+n+2)/8.at n=19A002817
• a(n) = p*(p-1)/2 for p = prime(n).at n=42A008837
• a(n) = floor(surface area of a sphere with radius n).at n=37A066644
• Triangular numbers whose reverse is prime.at n=12A066751
• Triangular numbers whose index is a multiple of the sum of their digits.at n=33A067520
• Numbers k such that phi((prime(k)-1)/2) = sigma(k).at n=41A068474
• Triangular numbers with property that digits alternate in parity.at n=29A068882
• Triangular numbers in which the k-th significant digit either divides k or is a multiple of k.at n=25A069559
• Numbers k such that gcd(3k,8^k+1) = 3 but k does not divide the numerator of B(2k) (the Bernoulli numbers).at n=26A070193
• Smallest triangular number k such that k-1 has exactly n (not necessarily distinct) prime factors.at n=9A081950
• Sort the digits of these triangular numbers into descending order and drop zeros to get primes.at n=26A082923
• Triangular numbers which are one more than a product of distinct triangular numbers.at n=14A083517
• a(n) = smallest k such that (10^k-1)/9 == 0 mod prime(n)^2, or 0 if no such k exists.at n=42A087094
• Triangle read by rows in which the n-th row contains the n smallest triangular numbers with the least significant digits of the n-th triangular number.at n=39A095225
• Least triangular number divisible by n-th prime.at n=42A112456
• Triangular numbers whose digit reversal is prime; trailing zeros are permitted.at n=17A115704
• Triangular numbers for which the sum of the digits is a prime number.at n=39A117512
• Numbers which are both lucky and triangular.at n=34A118565
• Triangular numbers congruent to 1 or 5 mod 6.at n=31A128880
• Quadruple lucky numbers (lower terms). Numbers n such that n, n+2, n+6, n+8 are all Lucky numbers.at n=17A139783