21991

Properties

Appears in sequences

• Primes that remain prime through 3 iterations of function f(x) = 2x + 9.at n=39A023276
• Primes that remain prime through 4 iterations of function f(x) = 2x + 9.at n=15A023306
• Primes that remain prime through 5 iterations of function f(x) = 2x + 9.at n=2A023334
• Primes such that applying "reverse and add" twice produces two more primes.at n=3A174402
• Number of nondecreasing arrangements of 4 numbers in -(n+2)..(n+2) with sum zero.at n=42A188212
• Number of nX7 0..1 arrays avoiding 0 0 0 and 0 1 1 horizontally and 0 1 1 and 1 1 0 vertically.at n=4A207487
• Number of 5Xn 0..1 arrays avoiding 0 0 0 and 0 1 1 horizontally and 0 1 1 and 1 1 0 vertically.at n=6A207490
• Number of (n+1)X(1+1) 0..2 arrays x(i,j) with row sums sum{j*x(i,j), j=1..1+1} nondecreasing, and column sums sum{i*x(i,j), i=1..n+1} nondecreasing.at n=6A232775
• T(n,k)=Number of (n+1)X(k+1) 0..2 arrays x(i,j) with row sums sum{j*x(i,j), j=1..k+1} nondecreasing, and column sums sum{i*x(i,j), i=1..n+1} nondecreasing.at n=21A232778
• T(n,k)=Number of (n+1)X(k+1) 0..2 arrays x(i,j) with row sums sum{j*x(i,j), j=1..k+1} nondecreasing, and column sums sum{i*x(i,j), i=1..n+1} nondecreasing.at n=27A232778
• Numbers k such that (209*10^k - 17)/3 is prime.at n=19A286176
• Index where n first appears in A381597.at n=35A381599
• Primes having only {1, 2, 9} as digits.at n=40A385776
• a(n) is the largest prime p such that b(n) = b(n-1)*(p+1)/(p-1) is an integer (A385959), where b(0) = 1.at n=38A385958
• One third the number of solid partitions of n with 5 parts.at n=30A387998
• Prime numbersat n=2463