18528

Appears in sequences

• a(n) = p*(p-1)/2 for p = prime(n).at n=43A008837
• a(n) = 49*(n*(n+1)/2) + 6.at n=27A061792
• a(n) = 25*n*(n + 1)/2 + 3.at n=38A061793
• Largest triangular number less than or equal to sum of previous terms with a(0)=1.at n=16A061883
• Triangular numbers whose index is a multiple of the sum of their digits.at n=34A067520
• a(1) = 8; a(n+1) = smallest triangular number > a(n) formed by adding at least one digit to a(n).at n=3A068623
• Rounded volume of a regular octahedron with edge length n.at n=34A071400
• Smallest multiple of (n+1)-st prime which is == 1 mod n-th prime.at n=42A073604
• Products of members of pairs in A075333.at n=33A075337
• Triangular numbers which are 7-almost primes.at n=10A076581
• a(n) = p(n)*(p(n)-1)/2 where p(n) = upper member of n-th pair of twin primes.at n=13A082669
• a(n) = smallest k where (10^k+1)=0 mod prime(n)^2, or 0 if no such k exists.at n=43A086981
• Least triangular number divisible by n-th prime.at n=43A112456
• Triangular numbers equal to the sum of a prime number with its index.at n=16A115886
• Triangular numbers that are sandwiched between two semiprimes; or triangular numbers t such that t-1 and t+1 are both semiprime.at n=9A121898
• Numbers k such that 9^k - 2 is a prime.at n=15A128455
• G.f.: A(x) = Product_{n>=1} [ (1-x)^2*(1 + 2x + 3x^2 +...+ n*x^(n-1)) ].at n=23A129355
• a(n) = 3*n*(6*n + 1).at n=32A144314
• Number of nX4 0..1 arrays avoiding the patterns 0 1 0 or 1 0 1 in any row, column, diagonal or antidiagonal.at n=6A206983
• Number of nX7 0..1 arrays avoiding the patterns 0 1 0 or 1 0 1 in any row, column, diagonal or antidiagonal.at n=3A206986