21218

Appears in sequences

• Consider the sequence of 4-tuples {0,a,b,c} (c>=a+b; a,b,c>0) which have the smallest integer 'c' required to reach {k,k,k,k} in n steps under map {r,s,t,u}->{|r-s|,|s-t|,|t-u|,|u-r|}. This sequence gives the third term 'b' of these quadruples.at n=27A034804
• Numbers that are not squarefree and whose Euler totient function is squarefree.at n=29A049198
• a(n) = 2*prime(n)^2.at n=26A079704
• Number of ways to toss a coin n times and not get a run of four.at n=16A135491
• 2*p^2, for p an odd prime.at n=25A143928
• a(n) = n^4 + 2^4 + (n+2)^4.at n=9A190176
• Composite numbers k such that Sum_{i=1..t-1} d(i+1)/d(i) is prime, where d(1), ..., d(t) are the divisors of k in ascending order.at n=26A255585
• Number of (n+2) X (1+2) 0..1 arrays with each 3 X 3 subblock having clockwise perimeter pattern 00000001 or 00000011.at n=11A259716
• Values A such that p=prime(n) satisfies binomial(2p-1, p-1) == 1 + A*p (mod p^4).at n=26A260209
• a(n) is the smallest composite k such that d(2)/d(1) + d(3)/d(2) + ... + d(q)/d(q-1) = prime(n), where d(1) < d(2) < ... < d(q) are the q divisors of k, or 0 if no such k exists.at n=28A260901
• Number of maximal matchings in the wheel graph on n nodes.at n=26A284709
• Where records occur in A070138.at n=27A298942
• a(n) = - Sum_{d|n} (-n/d)^d * binomial(d+n/d-2, d-1).at n=21A338688
• Number of marked chord diagrams (linear words in which each letter appears twice) with n chords, whose intersection graph is connected and distance-hereditary.at n=5A354588
• First differences of A000213, also twice A000073.at n=18A354784
• Numbers of the form 2*p^e, with p an odd prime and e >= 2.at n=40A354929
• Numbers whose second arithmetic derivative (A068346) is a primorial number (A002110) > 1.at n=33A368702
• Record high values of A379248.at n=58A379294