19306

Appears in sequences

• a(n) = p*(p-1)/2 for p = prime(n).at n=44A008837
• a(n) = Sum_{k=0..floor(n/2)} A026615(n-k,k).at n=20A026625
• a(n) = n^2*(n^2 + 1)/2.at n=14A037270
• The number phi_2(n) of Frobenius partitions that allow up to 2 repetitions of an integer in a row.at n=27A053993
• a(n) is the least multiple of n such that a(n) = 1 mod k for all integers k with 1 < k < n and k relatively prime to n.at n=13A055006
• Numbers n such that n | 7^n + 5^n + 3^n +1.at n=25A057830
• Triangular numbers whose digit permutations yield at least two further triangular numbers.at n=14A069674
• Triangular number x such that x + reverse of x is a prime.at n=7A072387
• Triangular numbers which are the sum of two squares.at n=29A073613
• Smaller of the two successive triangular numbers which differ in the use of only one digit.at n=34A077759
• a(n) = (2*n^3 - n^2 - n + 2)/2.at n=27A081441
• Sort the digits of these triangular numbers into descending order and drop zeros to get primes.at n=28A082923
• Triangular numbers which are one more than a product of distinct triangular numbers.at n=15A083517
• a(n) = smallest k such that (10^k-1)/9 == 0 mod prime(n)^2, or 0 if no such k exists.at n=44A087094
• Sum of next n numbers/n if n divides the sum else n times the sum of next n numbers.at n=13A094260
• Least triangular number divisible by n-th prime.at n=44A112456
• Triangular numbers whose digit reversal is a brilliant number (A078972).at n=8A115678
• a(n) = (n^4 + 46*n^3 - 169*n^2 + 146*n + 24)/24.at n=18A143059
• Number of planar triangular n X n X n nonnegative integer grids with every similarly oriented 4 X 4 X 4 subtriangle summing to 5.at n=17A154049
• Number of planar triangular n X n X n nonnegative integer grids with every similarly oriented 4 X 4 X 4 subtriangle summing to 5.at n=13A154049