21905

Appears in sequences

• Numbers k such that 80*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.at n=17A056695
• a(n) is the first term of the first run of exactly n non-perfect-powers.at n=46A087646
• a(n) = 16*n^2 + 1.at n=36A108211
• a(n) = 81*n^2 - 90*n + 26.at n=17A154295
• a(1)=3; for n > 1, a(n) = 1 + a(n-1) + gcd( a(n-1)*(a(n-1)+2), A073829(a(n-1)) ).at n=34A167053
• Position of the n-th prime in A253279.at n=45A255999
• Numbers of the form m^2 + 1 that can be expressed in more than one way as j^2 + k^2 with j > k > 1.at n=27A299708
• Numbers of the form m^2 + 1 that can be expressed in more than one way as j^2 + k^2 with j > k > 1 and gcd(j,k) = 1.at n=13A300166
• a(n) is the largest k such that the sum of k consecutive reciprocals 1/p_n + ... + 1/p_(n+k-1) does not exceed 1 (where p_n = n-th prime).at n=23A327600
• a(n) is the total number of squares after n iterations of the "Square Multiscale" substitution.at n=17A329919
• Main diagonal of A332365.at n=21A332366
• Column 2 of triangle in A288180.at n=13A333281
• a(n) is the smallest k > 1 such that 2^n - 2 divides k^n - 1, for n > 1.at n=16A340067
• G.f. satisfies A(x) = 1 + x*A(x) / (1 - x^2*A(x)^5).at n=10A365693
• For n >= 1, a(n) is the least k >= 0 such that k^2 + k + 1 is divisible by 2^n - 1 or a(n) = -1 if no such k exists.at n=16A372494