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 A213407 First occurrence of n in A213381. 2
 2, 0, 3, 5, 4, 9, 8, 7, 12, 17, 594270126787, 21, 20, 13, 24, 29, 16, 23, 32, 19, 36, 41, 1356, 45, 38, 151, 51, 411, 994, 57, 56, 31, 60, 65, 52, 69, 71, 37, 72, 77, 490, 81, 80, 169, 84, 6125, 68, 1043, 92, 49, 99, 101, 304, 105, 104, 55, 111, 63, 73, 89, 116, 61, 120, 125, 78, 129, 188, 67, 93, 137, 97, 18349355, 140, 433 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 LINKS FORMULA a(n) = smallest k>n such that (-2)^k == n (mod k+2). EXAMPLE Smallest n such that A213381(n)=9 is 17, so a(9)=17. PROG (C) #include // GCC -O3 -fopenmp #include   // 11 minutes #define SIZE 4096 #define STEP 0x1000000 long long first[SIZE]; int main(int argc, char **argv) {     unsigned long long a, i;     for (a=0; a0; p=(p*p)%b, t>>=1) {             if (t&1) r=(r*p)%b;         }         if (r

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Last modified November 30 18:40 EST 2021. Contains 349424 sequences. (Running on oeis4.)