21309

Appears in sequences

• Sums of successive Motzkin numbers.at n=12A005554
• Numbers k such that the period of the continued fraction for sqrt(k) contains exactly 78 ones.at n=28A031846
• Expansion of g.f.: 1/((1 - 2*x^2 + x^4 + 2*x^6 - x^8)*(1 - 2*x^2 - x^4 + 2*x^6 - x^8)).at n=24A147607
• Magic sums of 3 X 3 magic squares composed of primes in an arithmetic progression.at n=3A269324
• Odd numbers k such that phi(k) and cototient(k) have the same prime signature.at n=28A280927
• n/3 analog of Keith numbers.at n=5A282767
• Numbers k such that usigma(uphi(k)) = uphi(usigma(k)), where usigma is the sum of unitary divisors function (A034448) and uphi is the unitary totient function (A047994).at n=44A329730
• E.g.f.: Sum_{n>=0} x^n * exp( 2*x*exp(n*x) ) / n!.at n=6A340913
• Coefficient of x^n in the expansion of ( (1+x)^3 + x^3 )^n.at n=6A370287