20498

Appears in sequences

• Numbers k such that the continued fraction for sqrt(k) has odd period and if the last term of the periodic part is deleted the two central terms are both 40.at n=2A031628
• Numbers n such that 1n1, 3n3, 7n7 and 9n9 are all primes.at n=30A059677
• Number of paths from (0,0) to (n,n) avoiding 4 or more consecutive east steps and 4 or more consecutive north steps.at n=9A177792
• Number of (n+2) X (2+2) 0..3 arrays with every 3 X 3 subblock row and column sum not 2 3 6 or 7 and every diagonal and antidiagonal sum 2 3 6 or 7.at n=13A251888
• Number of (n+2) X (2+2) 0..3 arrays with every 3 X 3 subblock row and column sum not equal to 0 2 3 6 or 7 and every 3 X 3 diagonal and antidiagonal sum equal to 0 2 3 6 or 7.at n=13A252258
• Maximum starting value of X such that repeated replacement of X with X-ceiling(X/6) requires n steps to reach 0.at n=48A279076
• a(n) = Sum_{k=1..n} k^2*tau_3(k), where tau_3 is A007425.at n=17A319088
• a(n) = Sum_{k=1..n} tau(gcd(k,n))^(n/gcd(k,n)), where tau(n) is the number of divisors of n.at n=21A344195
• G.f.: Product_{k>=1} (1 + x^(2*k^2)) / (1 - x^k).at n=33A385009