Published online by Cambridge University Press: 20 November 2018
We study  
 $p$ -indivisibility of the central values  
 $L\left( 1,\,{{E}_{d}} \right)$  of quadratic twists  
 ${{E}_{d}}$  of a semi-stable elliptic curve  
 $E$  of conductor  
 $N$ . A consideration of the conjecture of Birch and Swinnerton-Dyer shows that the set of quadratic discriminants  
 $d$  splits naturally into several families  
 ${{\mathcal{F}}_{S}}$ , indexed by subsets  
 $S$  of the primes dividing  
 $N$ . Let  
 ${{\delta }_{S}}={{\gcd }_{d\in {{\mathcal{F}}_{S}}}}L{{(1,{{E}_{d}})}^{\text{alg}}}$ , where  
 $L{{(1,{{E}_{d}})}^{\text{alg}}}$  denotes the algebraic part of the central  
 $L$ -value,  
 $L(1,\,{{E}_{d}})$ . Our main theorem relates the  
 $p$ -adic valuations of  
 ${{\delta }_{S}}$  as  
 $S$  varies. As a consequence we present an application to a refined version of a question of Kolyvagin. Finally we explain an intriguing (albeit speculative) relation between Waldspurger packets on  
 $\widetilde{\text{S}{{\text{L}}_{2}}}$  and congruences of modular forms of integral and half-integral weight. In this context, we formulate a conjecture on congruences of half-integral weight forms and explain its relevance to the problem of  
 $p$ -indivisibility of  
 $L$ -values of quadratic twists.