30721

Appears in sequences

• Strong pseudoprimes to base 71.at n=17A020297
• Strong pseudoprimes to base 95.at n=12A020321
• Number of singular 2 X 2 matrices over Z(n) (i.e., with determinant = 0).at n=30A020478
• Number of noninvertible 2 X 2 matrices over Z/nZ (determinant is a divisor of 0).at n=29A020479
• a(n) = T(5,n), array T given by A048472.at n=10A048477
• a(n) = a(n-1) + a(m) for n >= 4, where m = 2*n - 2 - 2^(p+1) and p is the unique integer such that 2^p < n - 1 <= 2^(p+1), starting with a(1) = a(2) = 1 and a(3) = 3.at n=41A050035
• Composite numbers m such that phi(m)*sigma(m) is divisible by m-1.at n=36A065149
• a(n) = n + floor(Sum_{k<n} a(k)/2) with a(0)=0.at n=24A079719
• Diagonal in array of n-gonal numbers A081422.at n=30A081437
• Number of pairs with two different elements which can be obtained by selecting unique elements from two sets with n+1 and n^2 elements respectively and n common elements.at n=31A085490
• a(n) = n-th element of n-th row of triangle shown below.at n=15A115025
• Row sums of triangle A145364 (S1hat(-2)) and partition array A145363 (M31hat(-2)).at n=24A145365
• a(n) = 30*n^2 + 1.at n=32A158558
• a(n) = 15*2^(n+1) + 1.at n=10A195744
• Numbers n such that (n(n+1)/2) modulo sigma(n) = n.at n=14A232538
• E.g.f. satisfies: A'(x) = A(x)^6 / A(-x) with A(0) = 1.at n=5A235370
• Number of non-congruent solutions of x^2 + y^2 + z^2 + t^2 == 0 mod n.at n=30A240547
• Number of n X 4 0..1 arrays with each 1 adjacent to 0 or 3 king-move neighboring 1s.at n=6A295914
• Number of n X 7 0..1 arrays with each 1 adjacent to 0 or 3 king-move neighboring 1s.at n=3A295917
• T(n,k)=Number of n X k 0..1 arrays with each 1 adjacent to 0 or 3 king-move neighboring 1s.at n=48A295918